Reverts commit 43175fb. Pkg components no longer write vars in Makefile.
Add Component.phase_deps, definable by components.

The idea is to prevent components from resolving the flags of their
dependencies in As_project as is done now. We would like to resolve
them in backends. However we don't want to put too much logic in the
backends. The function takes a component and a phase it must return
the components whose corresponding phase flags should be injected in
the component's flags.
Add phase_deps to `Bin.

For a start.