[V3] Garbage collection - #1491

PR #1482 but rebased on v3

I've started on a proposal how we could implement garbage collection to clean up packages no longer needed in the packages folder (and if possible the same for paket-files). Addresses #1416.

Since the packages folder is typically excluded from version control, we do not always have explicit information about a package being removed. What we do know is what packages are currently extracted in the packages folder, and what packages we actually expect to be there according to the lock file.

The proposed algorithm essentially goes through the subdirs of the packages folder, tries to imply group and package names, and drops all of these folders which we identify as a extracted package but are not known by the lock file (ignoring the version).

Questions:

• Is it safe to assume that the name of the extracted nuspec file matches the package id, or do we need to parse it to be sure? We could also interpret the folder name instead, which we do construct ourselves (with or without version).
• When/where would we want to do this? Should it be explicit? Only on demand? Currently it is rather aggressive: always after add, remove and install, unless --no-install is set, and always after restore. I expect this to be too aggressive in practice.