Support Go modules

[negz]

Description of your changes

Write the generated files alongside the first Go file of the module, rather than to its presumed path under GOPATH. This change assumes:

• Packages always have at least one Go source file.
• The Go files of a package are always within a single directory.

I believe the above are always true.

Fixes #5

Checklist

I have:

• [x] Run make reviewable to ensure this PR is ready for review.
• [x] Ensured this PR contains a neat, self documenting set of commits.

[negz]

I've tested that this PR still works with an old fashioned $GOPATH setup, but have not yet tested that it works with Go modules.