Log Message:
go: Introduce support for GO_DEPS.

This supports packages that require a large number of go-based dependencies,
treating them as additional distfiles and built inline as part of the package
build.  The print-go-deps target is helpful to generate the list of GO_DEPS
required for each package by parsing the Gopkg.lock file in the extracted
sources.

Thanks to rillig@ for various suggestions and comments.