lib: Coerce flags enums to GIR bitfields

lib: Coerce flags enums to GIR bitfields

The GI scanner decides if an `enum` is really a `bitfield` if it finds
any values that have left shifts. With an `enumeration`, the
introspecting language may error or convert to a different type if the
user tries to combine values. Change all Flags `enum`s to use
left-shifted values so that they're represented as `bitfield`s in the
GIR.

The primary bug here is that you can't combine `REFS_ONLY` and
`NO_PRUNE` when calling `OSTree.Repo.prune()` from an introspected
language.

This is an IABI break since the typelib will change from `enumeration`
to `bitfield`. `OstreeRepoImportFlags` is internal but the change is
included here to prepare for a subsequent name that would require bit
shifting to operate correctly as a flag.