Use UnqualifiedQName for {Module,Submodule}Statement

UnqualifiedQName is the best approximation of a validated string
we have, even if it has a clunky name. Redefine module-like statements
to take that as an argument, forcing proper validation during
parse.

Change-Id: Ic902b8eedb0e955f00d14b41d115765c576df055
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>