Migrate package manager to backend

[citizenmatt]

This PR moves the code that calculates the list of in use packages from the frontend to the backend, allowing backend code to use the information. The data is passed to the fronted via the protocol, and is used to populate the Packages node of the Unity Explorer.

• Packages are read from packages-lock.json if it exists and is up to date. If not, it will read/fall back to manifest.json. The packages-lock.json file is created and maintained automatically by Unity, and is not expected to be modified by hand. The user can modify manifest.json by hand, and Unity will read this file, resolve packages and update packages-lock.json. If Unity isn't running, the file will get out of date, so up to date packages are read from manifest.json
• The Unity Explorer nodes have been refactored, mostly renaming and splitting into separate files
• Any file or folder that the IDE treats as ignored is now highlighted in the Unity Explorer with a "hidden folder" icon, or a text marker to say that it is ignored. This is especially important for Unity projects as Unity ships code in Samples~ and docs in Documentation~ and the IDE ignores *~ by default, meaning these files will not appear in Find in Files or work if opened as a solution.
• Reference nodes are no longer drawn using VCS status colours