Flag to constraint cabal-solver only to installed packages

I just realized that along with the more general #2568, a different specialization besides `--offline` (see #2566) would be even more convenient for me (and maybe others as well) to have: A flag `--no-dependencies` or `--only-installed` that would effectively limit the cabal solver to only what's already installed in the current package db. That's effectively how `cabal configure`/`cabal build` operate, which have to make do with what's already registered in the package database.