Duncan Coutts authored Aug 30, 2016 and Ben Gamari committed Aug 30, 2016

Previously interpreting the content of the .ghc.env files was done
after the step that loaded the available package dbs. This meant that
setting the package db flags was ineffective. This patch moves
interpreting the env files before loading of the package dbs.

Also, the package-db entries refer to files. Allow spaces in these file
names. Also treat as comments lines beginning with "--".

These are pretty minor fixes in a feature that up 'til now has been
essentially unused (witness no bug report about it), so there's very
low risk here. If we can get this into 8.0.2 then cabal can start
generating the .ghc.environment files, otherwise it cannot as it needs
the working package-db entries, to be able to refer to local package
dbs in the build tree (or cabal nix store).

Test Plan:
Manually create example .ghc.env files
run ghci; :show packages
Done this. It works.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2476