Ignore any specified profile when finding the Flatpak build target
    
    The source module for a Flatpak build can be specified as something like
    'flatpak-runtime:28/sdk' to indicate that the Flatpak is built from
    the 'sdk' profile of the flatpak-runtime module. This should be ignored
    when looking up module builds to find the platform stream and build
    target.
    
    Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>