Separate `cabal install --lib` into own command (cabal env)

... e.g. `cabal env`.

People do confuse what "installing libraries" and "installing executables" means, and after answering on few issues, I think stronger that separation of commands will make UX clearer.

An alternative is to drop `--lib` flag, and teach `cabal install` to do **the** right thing every time. I don't see how it would work.

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edit by @fgaz: I'm hijacking the OP to raise the visibility of the **cabal env prototype**: https://github.com/phadej/cabal-extras/tree/master/cabal-env
help & feedback are welcome

Start by reading the issue discussion and the cabal-env prototype linked in the edit. Review the existing cabal install behavior and the prototype's proposed command boundary before proceeding. Done means an agreed design and implementation that clearly separates library installation from executable installation.

Written by the indexing model from the issue text.