Local prebuilds

[jchook]

See #136 for discussion

Rationale

When an arch/platform/version does not have a published prebuild, we currently have two options:

1. Host an HTTP server with the proper directory structure to serve the missing prebuilds, and set node_config_${package}_binary_host in the build environment
2. Wait for CI flow to build the native modules

This PR enables users to provide prebuilds via the host's filesystem.

Changes

• Reinstates code removed by a069253
• Adds --local-prebuilds flag to specify prebuild dir
• Adds npm_config_${pkg.name}_local_prebuilds env var to specify prebuild dir
• Adds tests for local prebuilds

Notes

I mostly just re-added previously existing code. I also added:

• var + env config for the otherwise hard-coded prebuilds dir
• tests

[jchook]

Please give any feedback, esp suggestions on how to fix the Windows + Node 10 error on AppVeyor

[lovell]

Thanks for the PR, the CI failure appears to have been temporary as it worked when re-run. I've left one comment inline but otherwise this is looks good to me.

[jchook]

Thanks for the review 👍

[gochax]

Hi, when can this PR be merged? I would need its functionality quite urgently.

[MikeBauerCA]

This functionality would benefit our team as well! @lovell is there an intent to merge this PR in the near future? Thanks @jchook for opening 👍

[vweevers]

I don't have time to review and I'm on the fence about whether this feature is in scope, but this is a quality PR (has tests, good description, etc), I trust @lovell, and the upvotes indicate that multiple folks need this feature, so I'm merging and releasing.

[vweevers]

6.1.0