cmd/go: allow to specify modules to delete with 'go clean -modcache'

[hyangah]

go clean -modcache causes cleaning of the entire module download cache.

$ go1.13beta1 help clean
...
The -modcache flag causes clean to remove the entire module
download cache, including unpacked source code of versioned
dependencies.
...

This is useful when the module cache contains problematic modules
(e.g. module version that is different sum hash from what sum.golang.org has, etc) .
But, redownloading everything is sometimes costly.
It would be nice if users can have controls over what modules to delete.

What version of Go are you using (go version)?

$ go version
go1.13beta1

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

linux/amd64

[seankhliao]

Duplicate of #28835