- Support for prerequisite globbing [feature 0.5.0]

* Need a syntax to say "all excluding these".

  {*/ -build/ -doc/}
  {*/ - build/ doc/}

* There is POSIX glob(7) API.

* Probably both * (one level) and ** (recursive).

* Probably only support wildcards in one component.

* May make sense to provide generic support in libbutl since will probably
  be used in other places (e.g., subprojects globbing).