cabal install can be used inside a cabal exec environment

Inside a cabal exec environment cabal should be configured to always use the
correct environment. When there is a sandbox this is addressed by setting the
CABAL_SANDBOX_CONFIG environment variable.

However GHC is configured to use the correct package database through setting
the GHC_PACKAGE_PATH environment variable to include the sandbox database. The
Cabal library previously refused to operate when GHC_PACKAGE_PATH is set in
order to avoid having a different view of the package databases to GHC.

In the case of a cabal exec environment being loaded for a cabal sandbox, it
is safe to allow the use of GHC_PACKAGE_PATH as it is being used to ensure
that GHC uses the same package database as cabal does.

A check is made that GHC_PACKAGE_PATH matches the value that cabal exec set it
to. If it does use of GHC through cabal is permitted.

Fixes #1800
Merge branch 'use-cabal-inside-cabal-exec' of https://github.com/benarmston/cabal
-Wall police