Copy package to Packages folder to make editable

If trying to edit a file from a read only package (e.g. something referenced from `manifest.json`), prompt to copy the package to the Packages folder and make editable. If accepted, copy the entire package folder from its current location (i.e. `Library/PackageCache`) to the `Packages` folder. Also make sure the files are no longer read-only.

Care needs to be taken to:
* Copy everything in the background
* Once complete, notify Unity to refresh the asset database. This should rebuild projects and include the new folder as one or more projects
* Close currently open files from that package to avoid confusion?
* What happens to the existing cached folder?

Start by tracing how read-only files referenced from manifest.json are handled, along with package locations in Library/PackageCache and Packages. Define the copy, background operation, read-only removal, Unity asset-database refresh, open-file handling, and cached-folder behavior; completion requires these cases to be resolved and implemented.

Written by the indexing model from the issue text.