New Command: Delete UPM Cache

[chrisyarbrough]

When developing packages, it's sometimes helpful clear all caches that Unity Package Manager stores on a system.

Issue causes:

• Somebody (incorrectly) deletes a package version from a custom registry, then republishes the same version number. Now all clients must delete all caches to receive the new package or they might face PacMan errors.
• When switching git revisions in a repo, Unity Library state can sometimes get corrupted (old and new package versions fighting), this requires clearing the project package cache.

Cache locations:

• In a project
• On the computer (multiple cache locations) https://docs.unity3d.com/6000.3/Documentation/Manual/upm-cache.html
• Optionally, support custom cache location: https://docs.unity3d.com/6000.3/Documentation/Manual/upm-config-cache.html