Refactor: promote local-only get_schema_path to internal pcmk__ variant

This function is to be utilized more widely. At that occasion, extend
the scope of refactoring to also generalize the only predestined path
(symbolically: /xml) to more "namespaces" to allow for gradual move
of the XML-related artefacts from there (referred to as "legacy"
namespace since this commit) to fine-grained subdivisions like already
started with /xml/api (since b908ae456), having CIB related stuff and
upcoming ACLs-rendering-in-cibmon specifically in mind. Since we
thusly shift towards more regularities, as a cherry on the cake, we
drop redundant information tracked per each pacemaker schemas, since
the refactored function only takes a singular input (beside the
namespace indication) in place of original two (mutually correlated).