Use a non-default build dir when building add-source deps.

Fixes #1281.
Merge pull request #1312 from 23Skidoo/sandbox-dist-builddir

Use a non-default build dir when building add-source deps.
80-col violations.
Use a subdir of 'dist' as build dir for building add-source deps.

Works better with 'cabal clean'.
80-col violations.
Make configAbsolutePaths work with sandboxes.

The package DB dir is created *after* we've changed the 'configPackageDBs'
setting with 'setPackageDB'.

Aside: we really need some functional tests for sandboxes.
80-col violations.
Typos.