modules: add mod clean -modcache or similar

[myitcv]

In the spirit of go clean -modcache: https://go.dev/ref/mod#go-clean-modcache

More detail to follow

[mvdan]

My 2c: the only reason Go ended up with flags like -cache and -modcache is that go clean used to be for cleaning build artifacts from packages. Nowadays, almost noone uses it for that purpose anymore:

Clean removes object files from package source directories.
The go command builds most objects in a temporary directory,
so go clean is mainly concerned with object files left by other
tools or by manual invocations of go build.

I don't think we will ever have a need for such a "clean package build files" command. Even if evaluating a CUE package or building its e.g. wasm artifacts were to produce some sort of build output, that would be in the cache, not in the very same package directory.

For that reason, I think we should go for something like cue clean modcache, i.e. an argument instead of a flag. Given that all of our caches live under the same directory, we could also provide cue clean allcaches to delete all of it.

It's also worth noting that "modcache" is an artifact from Go's GOMODCACHE env var, which we do not have, so we can likely come up with a better name like go clean modulecache or, as a hierarchy, go clean cache/modules.

[mvdan]

or, as a hierarchy, go clean cache/modules.

note that this prompted me to file #3139.