Solver: Enforce dependencies on executables (fixes #4781).

This commit adds two checks to the validation phase of the solver:

1. It checks that each newly chosen package instance contains all executables
   that are required from that package so far.

2. It checks that each new build tool dependency that refers to a previously
   chosen package can be satisfied by the executables in that package.

This commit also fixes a TODO related to solver log messages. Previously, it was
possible for the log to associate an incorrect executable name with a
dependency.