Remove EXTERNAL_SET_DEPENDENCY machinery.

It didn't make any sense to have that in there. If a subproject
wants to import something that uses Google Test and Google Mock
then it has to guaruntee that it is built before it is built. E.g,

 - superproject
   - gmock(a)
   - subproject (depends on gmock)
     - gmock(b) (imported from a)
     - subsubproject
       - gmock(c) (imported from a)