- bpkg create amalgamation functionality in build2 [feature 0.5.0]

* Would be nice to have wildcard prerequisite (with exclusion) support for
  the root buildfile.

* We can also pre-load (or pre-init) the specified modules and decide where
  to put them (in bootstrap.build if they are bootable).

* Naturally, bpkg should then use this instead of doing it itself.

* Probably makes sense to combine with configure somehow.