Fully reset incremental state on clean

[jozanek]

Fixes #1466.

Converting a package foo.bar declaration into a top-level object bar of the same name reported bar is already defined as package bar. The error survived "Clean compiling workspace", restarting the build server, and reimporting the build — only rm -rf .bloop cleared it.

Cause

clean deleted the class directories but kept two pieces of incremental state:

• the persisted Zinc analysis file (<project.out>/<name>-analysis.bin), and
• the in-memory CompileGatekeeper last-successful result.

After a restart the stale analysis was reloaded and the next compile ran incrementally against it, so the old package foo.bar definition kept colliding with the new object bar. rm -rf .bloop was the only fix because it also removes the analysis file.

Note for reviewers: the analysis file is now deleted for all cleaned targets, not only those with an in-memory successful result — the file can exist on disk even when the in-memory cache has none (e.g. right after a restart), so it can't reuse the existing successful-keyed deletion loop.