speed up multiple invocations of counterfeiter

[joefitzgerald]

In v6, we switched to use go/packages to load interfaces and packages, which gave us compatibility with go modules. It also included a significant degradation in speed that was mostly tolerable for single invocations of counterfeiter, but unacceptable when multiple invocations were required (see #120).

This PR implements two things:

1. a very simple cache that takes the work done by packages.Load and caches the results
2. logic that will identify all //go:generate directives in a given package; for a package with 100 directives
   • go generate will invoke counterfeiter 100 times
   • on the first invocation, counterfeiter will generate fakes for all 100 directives (leveraging the cache)
   • on invocations 2..100, counterfeiter will be a no-op and exit without doing work

This yields impressive performance gains, but is still a regression relative to v5 performance (an approach which will not work for go modules), using go generate in https://github.com/cloudfoundry/cli/tree/7dc650d65b462a1df674b5697121b08362003cd0/cf/api as a benchmark:

• counterfeiter v6 (with caching) (3.92 seconds) runs about 20.5 times faster than v6 (without caching) (80.45 seconds)
• counterfeiter v6 (with caching) (3.92 seconds) runs about 5 times slower than v5 (0.78 seconds)

counterfeiter v5

$ time go generate
Wrote `FakeAppSummaryRepository` to `apifakes/fake_app_summary_repository.go`
Wrote `FakeBuildpackBitsRepository` to `apifakes/fake_buildpack_bits_repository.go`
Wrote `FakeBuildpackRepository` to `apifakes/fake_buildpack_repository.go`
Wrote `FakeClientRepository` to `apifakes/fake_client_repository.go`
Wrote `FakeCurlRepository` to `apifakes/fake_curl_repository.go`
Wrote `FakeDomainRepository` to `apifakes/fake_domain_repository.go`
Wrote `FakeRouteServiceBindingRepository` to `apifakes/fake_route_service_binding_repository.go`
Wrote `FakeRouteRepository` to `apifakes/fake_route_repository.go`
Wrote `FakeRoutingAPIRepository` to `apifakes/fake_routing_apirepository.go`
Wrote `FakeServiceAuthTokenRepository` to `apifakes/fake_service_auth_token_repository.go`
Wrote `FakeServiceBindingRepository` to `apifakes/fake_service_binding_repository.go`
Wrote `FakeServiceBrokerRepository` to `apifakes/fake_service_broker_repository.go`
Wrote `FakeServiceKeyRepository` to `apifakes/fake_service_key_repository.go`
Wrote `FakeServicePlanRepository` to `apifakes/fake_service_plan_repository.go`
Wrote `FakeServicePlanVisibilityRepository` to `apifakes/fake_service_plan_visibility_repository.go`
Wrote `FakeServiceSummaryRepository` to `apifakes/fake_service_summary_repository.go`
Wrote `FakeServiceRepository` to `apifakes/fake_service_repository.go`
Wrote `FakeUserProvidedServiceInstanceRepository` to `apifakes/fake_user_provided_service_instance_repository.go`
Wrote `FakeUserRepository` to `apifakes/fake_user_repository.go`
go generate  0.78s user 0.29s system 126% cpu 0.844 total

counterfeiter v6 (caching disabled)

$ export COUNTERFEITER_DISABLECACHE=true
$ time go generate
Writing `FakeAppSummaryRepository` to `apifakes/fake_app_summary_repository.go`... Done
Writing `FakeBuildpackBitsRepository` to `apifakes/fake_buildpack_bits_repository.go`... Done
Writing `FakeBuildpackRepository` to `apifakes/fake_buildpack_repository.go`... Done
Writing `FakeClientRepository` to `apifakes/fake_client_repository.go`... Done
Writing `FakeCurlRepository` to `apifakes/fake_curl_repository.go`... Done
Writing `FakeDomainRepository` to `apifakes/fake_domain_repository.go`... Done
Writing `FakeRouteServiceBindingRepository` to `apifakes/fake_route_service_binding_repository.go`... Done
Writing `FakeRouteRepository` to `apifakes/fake_route_repository.go`... Done
Writing `FakeRoutingAPIRepository` to `apifakes/fake_routing_apirepository.go`... Done
Writing `FakeServiceAuthTokenRepository` to `apifakes/fake_service_auth_token_repository.go`... Done
Writing `FakeServiceBindingRepository` to `apifakes/fake_service_binding_repository.go`... Done
Writing `FakeServiceBrokerRepository` to `apifakes/fake_service_broker_repository.go`... Done
Writing `FakeServiceKeyRepository` to `apifakes/fake_service_key_repository.go`... Done
Writing `FakeServicePlanRepository` to `apifakes/fake_service_plan_repository.go`... Done
Writing `FakeServicePlanVisibilityRepository` to `apifakes/fake_service_plan_visibility_repository.go`... Done
Writing `FakeServiceSummaryRepository` to `apifakes/fake_service_summary_repository.go`... Done
Writing `FakeServiceRepository` to `apifakes/fake_service_repository.go`... Done
Writing `FakeUserProvidedServiceInstanceRepository` to `apifakes/fake_user_provided_service_instance_repository.go`... Done
Writing `FakeUserRepository` to `apifakes/fake_user_repository.go`... Done
go generate  80.45s user 33.72s system 380% cpu 29.985 total

counterfeiter v6

$ time go generate
Writing `FakeAppSummaryRepository` to `apifakes/fake_app_summary_repository.go`... Done
Writing `FakeBuildpackBitsRepository` to `apifakes/fake_buildpack_bits_repository.go`... Done
Writing `FakeBuildpackRepository` to `apifakes/fake_buildpack_repository.go`... Done
Writing `FakeClientRepository` to `apifakes/fake_client_repository.go`... Done
Writing `FakeCurlRepository` to `apifakes/fake_curl_repository.go`... Done
Writing `FakeDomainRepository` to `apifakes/fake_domain_repository.go`... Done
Writing `FakeRouteServiceBindingRepository` to `apifakes/fake_route_service_binding_repository.go`... Done
Writing `FakeRouteRepository` to `apifakes/fake_route_repository.go`... Done
Writing `FakeRoutingAPIRepository` to `apifakes/fake_routing_apirepository.go`... Done
Writing `FakeServiceAuthTokenRepository` to `apifakes/fake_service_auth_token_repository.go`... Done
Writing `FakeServiceBindingRepository` to `apifakes/fake_service_binding_repository.go`... Done
Writing `FakeServiceBrokerRepository` to `apifakes/fake_service_broker_repository.go`... Done
Writing `FakeServiceKeyRepository` to `apifakes/fake_service_key_repository.go`... Done
Writing `FakeServicePlanRepository` to `apifakes/fake_service_plan_repository.go`... Done
Writing `FakeServicePlanVisibilityRepository` to `apifakes/fake_service_plan_visibility_repository.go`... Done
Writing `FakeServiceSummaryRepository` to `apifakes/fake_service_summary_repository.go`... Done
Writing `FakeServiceRepository` to `apifakes/fake_service_repository.go`... Done
Writing `FakeUserProvidedServiceInstanceRepository` to `apifakes/fake_user_provided_service_instance_repository.go`... Done
Writing `FakeUserRepository` to `apifakes/fake_user_repository.go`... Done
go generate  3.92s user 1.66s system 322% cpu 1.731 total

/cc @Samze @jamesjoshuahill @williammartin