Work with GHC_PACKAGE_PATH

At the moment, Cabal will die if run with the `GHC_PACKAGE_PATH` environment variable set. I think this is an arbitrary restriction. Instead, it should
1. Parse the search path in `GHC_PACKAGE_PATH`,
2. treat these paths as if they were passed to the `configure` command by `--package-db`, and
3. unset `GHC_PACKAGE_PATH` before invoking GHC.

Cabal should do the same for `GHCJS_PACKAGE_PATH`.

See also #2711.

I am implementing this in `reconfigure`.

/cc @ezyang @23Skidoo