Test multiple file support when files are distributed over
different directories.

Dependency graph

   a/a.v
     \
      \
      b/b1.v --- b/b2.v
                   \
                    \
                    c/c.v

That is, c depends on b2, which depends on b1, which depends on
a.

Script c.v and watch the compilation messages in *Messages*.
Touch any file and retract and assert the Require in c.