Improve `--prefix` support

This PR improves `--prefix` support for `enpkg`.

It builds off of [Puneeth's branch](https://github.com/punchagan/enstaller/commits/prefix-based-cache), which fixed the cache directory.

In addition, this verifies that prefixes lead to site-packages paths on PYTHONPATH and the egg-index prefers eggs in the user-supplied prefix over those in default prefix. (This assumes that the user-supplied prefix is earlier in the python path than the default prefix, which seemed to be what some other code assumed.)