wraps from subprojects are now merged into the list of wraps from main
project, so they can be used to download dependencies of dependencies
instead of having to promote wraps manually. If multiple projects
provides the same wrap file, the first one to be configured wins.

This also fix usage of sub-subproject that don't have wrap files. We can
now configure B when its source tree is at
`subprojects/A/subprojects/B/`. This has the implication that we cannot
assume that subproject "foo" is at `self.subproject_dir / 'foo'` any
more.

A `#` is the root-prompt, whereas `$` is the user-prompt.

Upper or lower case depending on the official spelling, or the more
consistent usage.

This allows us to more easily have the documentation in sync with
the source code as people will have to document new features etc
right at the time where they implement it.