fix: fix a bug where '.' strip prefixes were not extracted correctly

[kormide]

This addresses the case where people use pkg_tar to produce their release archive, which creates a . prefix by default: bazelbuild/rules_pkg#50

[alexeagle]

nit: seems like this assignment should be in an else case so modulePath could be const?

[kormide]

For modulePath to be const I think have to use a condition ? a : b clause since const variables can't be undefined. But yeah I agree an if else block would be cleaner. Will fix.

[kormide]

I'm having second thoughts on this solution. It's an issue with rules_java but that ruleset can already be consumed by Bazel without setting a strip_prefix of ".". If that's the case, then I don't think we should ask people to add that prefix and have it committed in the BCR entry.

I think the better approach would just be to silently handle this case and make it equivalent to having an empty strip prefix.