cmd/cue: the directory tree structure of the modules cache is inconsistent

[mvdan]

After cleaning my entire ~/.cache/cue and running cue mod tidy on a module which depends on another module, I see:

$ find .cache/cue -type f
.cache/cue/cache/download/github.com/mvdan/test-repo/@v/v0.0.11.lock
.cache/cue/cache/download/github.com/mvdan/test-repo/@v/v0.0.11.zip
.cache/cue/cache/download/github.com/mvdan/test-repo/@v/v0.0.11.mod
.cache/cue/github.com/mvdan/test-repo@v0.0.11/cue.mod/module.cue
.cache/cue/github.com/mvdan/test-repo@v0.0.11/foo.cue

• ~/.cache/cue/cache/download is unnecessarily repetitive; we should not insert a "cache" directory inside... CUE's cache directory.
• The unzipped modules are extracted straight inside CUE's cache directory, causing namespace issues for future kinds of caches we might want to use.

My thinking is that we should use ${CUE_CACHE_DIR}/mod/download/${modpath} for downloaded artifacts (e.g. zip files), and ${CUE_CACHE_DIR}/mod/extract/${modpath} for the contents of extracted archives. This way they are nicely grouped together, and there is some hierarchy that describes their contents as well.