Save timestamps of add-source dependencies in 'sandbox add-source'.
80-col violations.
Refactor 'prepareTree'.

Factor out the parts that are used to list all source files mentioned in a
package description.
Move timestamp functions to their own module.
80-col violations.
Make 'inDir' a bit more polymorphic.

The change doesn't affect existing code.
Actually implement the timestamp check in 'reinstallAddSourceDeps'.
Replace a conditional with a guard.
Use maybeToList.
Comment.
Merge pull request #1292 from 23Skidoo/minimal-rebuilds

Minimal rebuilds for add-source dependencies