Refactor: libcrmcommon: add pcmk__is_clone()

... and use it instead of pe_rsc_is_clone() from libpe_status.

This is internal API so that it can remain inline even after pcmk_resource_t
becomes an opaque type. When that happens, pcmk_resource_t should have an
accessor function for variant, so external code can compare that against
pcmk_rsc_variant_clone.