Reinstall modified add-source deps in 'install' if "." is among the targets.

Fixes #1265.
Formatting.
Update cabal.sandbox.config when doing 'install .'.

So that we don't reinstall add-source deps for the wrong compiler (e.g. when
doing 'cabal build' after 'cabal install -w'). We already do this in configure,
and running 'install .' means configuring the current package.