Add support for generating go module deps

[cpuguy83]

This adds a new Generate field on Source which is a list of
generator configs.
Today the only type of generator we support is for go modules but we'll
likely need to add support for other things, e.g. Rust cargo deps.

Multiple geneator configs are allowed in the case of a source with
multiple go modules defined in it.

Closes #180

[cpuguy83]

I've updated this with a slightly different implementation that is more extensible and easier to discover.
Also marked this ready for review.

[DannyBrito]

I am a bit confused on how this works, so this whole PR would allow for any specified source to have in this case a gomod generator. So for a scenario that we have at least 2 sources that have gomod gen. When we retrieve these gomods do these get merged into a single directory and state for all the gomods for the different sources? or each gomod would have it's own state? The other relevant section for this question is when preparing the sources in the rpm template: frontend/rpm/template.go

	prepareGomods := sync.OnceFunc(func() {
		if !w.Spec.HasGomods() {
			return
		}
		fmt.Fprintf(b, "mkdir -p \"%%{_builddir}/%s\"\n", gomodsName)
		fmt.Fprintf(b, "tar -C \"%%{_builddir}/%s\" -xzf \"%%{_sourcedir}/%s.tar.gz\"\n", gomodsName, gomodsName)
	})

the gomodsName is a constant so it doesn't make sense to do this more than once if the gomod modules are merged into one for all sources.

[cpuguy83]

When we retrieve these gomods do these get merged into a single directory and state for all the gomods for the different sources?

Yes, that is what this function is doing.
The modules are stored in the state referenced by the deps variable, which is getting overwritten below (L90) with deps = deps.With(withMods), where withMods is an llb.StateOption that modifies the state to include the deps for an individual source.

The trick there is in the withGomod function where it is using return value of llb.Run(...).AddMount(gomodCachedir, in).
Here the return value of AddMount is the the content of the mount point after the Run has executed.

[cpuguy83]

Updated this, also made sure to patch any go mod sources before fetching deps in case go.mod or go.sum are being patched.

[adamperlin]

I think this looks good! It would be interesting to see how this could possibly be slotted into an interface as described in #213