modelbuilder: name the main module like the first module.

It is the way done by the compilers, and a lot of parts use
`mainmodule.name` to name things when `-m` is used.

So, in order to not broke the compilers parts,
generalize the idea in phases.

Note: the implicit main-module have no projets associated, so things
should not use "<main>" to identify then.

Signed-off-by: Jean Privat <jean@pryen.org>
abs_comp: move call of `supports_libunwind` in the right place

Signed-off-by: Jean Privat <jean@pryen.org>