Support --assume-installed in bundle config files

[achadwick]

It would be nice to allow users to exclude particular packages from installation into the bundle. This may be because their app might not need the functionality of one package pulled in as a dependency, or from a dependency chain.

(Aside: this is exacerbated by MSYS2's habit of making fat packages with everything thrown in including dev stuff. For example, my test mypaint.cfg eventually pulls in Python 3, despite MyPaint being a Python 2 program still.)