Problem: Currently it is possible to have plugin in a "partial install"
state when `git clone` was successfull but `git checkout` was not.
This was done to not checkout default branch by default in these
situations (for security reasons).
The problem is that it adds complexity when both dealing with lockfile
(plugin's `rev` might be `nil`) and in how `src` and `version` are
treated (wrong `src` - no plugin on disk; wrong `version` - "partial"
plugin on disk).
Solution: Treat plugin as "installed" if both `git clone` and
`git checkout` are successful, while ensuring that not installed
plugins are not on disk and in lockfile.
This also means that if in 'init.lua' there is a `vim.pack.add()` with
bad `version`, for first install there will be an informative error
about it BUT next session will also try to install it. The solution is
the same - adjust `version` beforehand.
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