For non-Custom packages, replace sdist with hand-rolled rebuild checking.
New module Distribution.Client.SourceFiles implements 'needElaboratedConfiguredPackage', which if run in the 'Rebuild' monad is sufficient to ensure all source files that participate in a build are monitored. Fixes #3401. It also fixes the "we didn't detect a new file appearing" problem. Signed-off-by: Edward Z. Yang <email@example.com>