Refuse to convert stls in Packages because the directory may be immutable

[mrpropellers]

Proposed change(s)

PickAndPlace integration tests failed because on installing URDF Importer via the package manager, the stl file in the Tests folder is detected and processed which results in URDF importer attempting to save a prefab in a directory that can't be written to.

Changed the logic in the asset post processing to ignore assets if they are not in the "Assets" folder (aka not a part of the currently open Project).

Useful links (GitHub issues, JIRA tickets, forum threads, etc.)

https://unity-ci.cds.internal.unity3d.com/job/7107835

Types of change(s)

• [x ] Bug fix

Testing and Verification

Validated locally that package assets are ignored by copying new stls into different locations.

Checklist

• [x ] Ensured this PR is up-to-date with the dev branch
• [x] Created this PR to target the dev branch
• [x] Followed the style guidelines as described in the Contribution Guidelines
• [x] (N/A) Added tests that prove my fix is effective or that my feature works
• [x] Updated the Changelog and described changes in the Unreleased section

[mrpropellers]

It would have been nice to include a test to reduce the likelihood of a regression bug. Not quite sure what would be a good one though.

In a sense we have this test, since this bug was caught on Yamato. I'm not sure we can do something more targeted than that in this repo, since the bug only appeared when importing the package via git checkout rather than local path.