[build2] Build system module for code-generation utilities Qt moc, etc.

[Sam Schweigel]

Boris Kolpackov writes:

> It seems the clean way to do it would be to define "Qt exe" and
> "Qt lib" target types that would "signal" that such additional
> functionality is required. For example:
>
> qtexe{hello}: {hxx cxx}{main}

I see how this would be implemented, but I worry that this wouldn't
compose well with other code generation tools.  What if the user wanted
to use another tool on the same executable that needed to find
dependencies in a similar way?

I'm going to see how far I can get by explicitly specifying the
`moc.cxx` targets for now.

It will be exciting to see how work on dynamically-loadable build
modules progresses.