Handle Bzlmod qualified repo names in coursier unpinned repo implementation

[plobsing]

Under Bzlmod, repo names are qualified. Whereas a simple workspace with a single
@maven repo would have before had an unpinned repo named @unpinned_maven, now
this repo is named something like @rules_jvm_external~~maven~unpinned_maven.

Using str.rpartition by the ~ delimiter enables us to consistently
get the unqualified name in both cases:

>>> "rules_jvm_external~~maven~unpinned_maven".rpartition("~")
('rules_jvm_external~~maven', '~', 'unpinned_maven')
>>> "unpinned_maven".rpartition("~")
('', '', 'unpinned_maven')

After this change, the lockfile pinning script, @maven//:pin will use the local
machine's Coursier cache (~/Library/Caches/Coursier on MacOS) even under
Bzlmod. A populated cache makes version resolutions substantially faster to obtain.

[Wyverald]

Please try not to rely on the canonical repo name format, which is unstable. In fact, we're looking to change the ~ to + to address bazelbuild/bazel#22865. Instead, you could explicitly pass in the "unpinned-ness" of a repo as a boolean attribute.

[plobsing]

Please try not to rely on the canonical repo name format, which is unstable. In fact, we're looking to change the ~ to + to address bazelbuild/bazel#22865. Instead, you could explicitly pass in the "unpinned-ness" of a repo as a boolean attribute.

Fair enough. I wasn't really a fan of the string munging going on, but ignored it to get the smallest patch that would solve my issue. Your way does seem cleaner to me, if we want to do that, its here: #1204 .
Demoting this PR to draft while that one is in flight (will close if it merges).