GHC doesn't have a way to ask for user-package-db, so Cabal reimplemented it.

Authored by ezyang.

Description

GHC doesn't have a way to ask for user-package-db, so Cabal reimplemented it.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: docs only

Reviewers: austin, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1644