Allow an override to the package spec
Allow an override to the package spec

This is useful if you have a "monorepo" that contains multiple ./cmd
dirs and you have multiple apps and want to compile only a subset of the
commands for each app.

heroku/heroku-buildpack-go,cloudfoundry/go-buildpack,heroku/heroku-buildpack-go,cloudfoundry/go-buildpack,michael-go/heroku-buildpack-go,titanous/heroku-buildpack-go,cloudfoundry/go-buildpack,michael-go/heroku-buildpack-go