fileset: reorder 'or' expression by weight

fileset: introduce weight constants for readability These constants are defined in the filesetlang module since it's the bottommost module depending on WEIGHT_CHECK_FILENAME, and extensions will be likely to import it to process function arguments. Credit for the naming goes to Augie Fackler.

sparse: use named parameters in i18n strings This should give more hints about what the %s means, and allow reordering.