modelbuilder: add parameter `given_mmodules` to `Phase::process_mainmodule`

modelbuilder: add parameter `given_mmodules` to `Phase::process_mainmodule`

`given_modules` is the list of explicitely requested modules.
From the command-line for instance.

Thus, global phases can chose to work with the bottom main module or
with each specific given modules.

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