Add a workaround to allow older cabal-install to use ghc-7.10

Summary:
This should smooth the upgrade process for people and help with testing
the 7.10 RCs. Otherwise people need to first install cabal-install-1.22
before they can use 7.10.

The problem is that older cabal still used file-style package dbs for
the inplace package db when building packages. The workaround is that
both ghc and ghc-pkg will notice when cabal tells them to use a file
style db e.g. "dist/package.conf.inplace" and, so long as that db is
empty (ie content is []) then they'll instead us a dir style db with
the same name but ".d" appended, so in this example that would be
"dist/package.conf.inplace.d". We have to use a separate dir rather
than transparently upgrading because old Cabal really assumes the path
is a file, and if it encounters a dir it will fail.

This seems to be enough for older Cabal to work, and may well be enough
for other scripts that create dbs using "echo [] > package.conf".

Test Plan:
validate and check new and old cabal can sucessfully install things,
including packages that have internal deps (ie using the inplace db)

Reviewers: hvr, tibbe, austin

Reviewed By: tibbe, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D628

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