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

[Sam Schweigel]

Hello,

I've had a lot of success packaging libraries for build2 so far, and I'd
like to experiment with getting Qt5 building using it, which involves a
number of preprocessing steps (moc, rcc, uic).  I understand that these
would be implemented as build system modules, and that eventually the
plan is to have dynamically-loadable build system modules that would be
packaged with the build tools themselves.  To get a feel for the build2
codebase, I've tried to implement a `moc` rule myself:

using cxx moc

exe{hello}: cxx{file1} hxx{file1} cxx{file1_moc}
cxx{file1_moc}: hxx{file1}

Where the `moc` module introduces no target types and contains a rule
that can turn an `hxx{xyz}` into an `cxx{xyz_moc}`.  Is this the
idiomatic way of doing this?

Additionally, I'd like to be able to scan all the `hxx` targets for the
string `Q_OBJECT`, and inject all targets that depend on that header
with a dependency on `cxx{blah_moc}`.  Where should the logic for this
go?

Thank you for all your hard work on build2,

Sam