mmd_resolver: rework solvables generation
    
    Now we have good separation between runtime and buildtime dependencies:
    If mmd has context, it is built artefact which supposed to have only one
    dependency entry, we create solvable with real arch for it. Otherwise we
    create multiple solvables with "src" arch.
    
    In "src" solvables, "context" is a number which represents index in
    dependencies array.
    
    Stream is not included anymore in "evr" because it's incomparable.
    Version is not included anymore in "name" because it's there just for
    comparison, nothing more (it's not different module-stream).
    
    As a side, add_requires/add_conflicts/add_provides were replaced with
    non-deprecated add_deparray method.
    
    With all this, we are ready to handle MMDs which have multiple elements
    in dependencies (modulemd v2).
    
    Signed-off-by: Igor Gnatenko <ignatenko@redhat.com>