The importers are now only responsible for reading config files and
feeding unvalidated lock and constraint hints into importPackages.

* Don’t throw away a locked version match due to a conflicting
  constraint unless there is another match that _does_ match
* Ignore pinned constraints
* Ignore conflicting constraints

github.com/carolynvs/deptest-importers contains the entire matrix of
testdata needed by the importers

* The setup and validation for all the importer testcases are the same,
  they just need to handle calling the importer properly.
* Flag importer tests to run in parallel now that they've been cleaned up.

The closure created by t.Run wasn't copying the testcase value, so the
test was being run on the wrong values.

When a lock hint is empty, nothing should be added to the lock