fix: cache clear now deletes dep files from disk, not just JSON

[Bhumika-SN]

Problem

jbang cache clear was not actually clearing the dependency cache from disk.

The DependencyCache.clear() method only set the in-memory depCache to null,
but never deleted the actual cached files on disk. This means running
jbang cache clear would leave all downloaded JARs and the dependency_cache.json
file still present on disk.

Fix

Updated DependencyCache.clear() to also:

• Delete the dependency_cache.json file from disk
• Delete the depcache directory containing actual downloaded JARs

Related Issue

Fixes #475