## Summary
Fixes#16637. (I hit this bug after 11h of work on a long-running task.)
Plugin cache initialization could panic when an already-absolute cache
path was normalized through `AbsolutePathBuf::from_absolute_path`,
because that path still consulted `current_dir()`.
This changes absolute-path normalization so already-absolute paths do
not depend on cwd, and makes plugin cache root construction available as
a fallible path through `PluginStore::try_new()`. Plugin cache subpaths
now use `AbsolutePathBuf::join()` instead of re-absolutizing derived
absolute paths.
Eric Traut