Regularise discovery for fragment-identified standards

[msdemlei]

In the Servicetype constraint (in registry.rtcons), only “old-style” standardIDs (which are references to full registry records) are supported without hacks. “New-style” references like SIA2's (e.g., ivo://ivoa.net/std/sia#query-2.0) require extra code, partly because the mapping is not as trivial as when we currently build SERVICE_TYPE_MAP, partly because we cannot compare with equality but need to RE-compare to ignore the minor version.

Since we now have three or the new-style standard_ids (see Servicetype.__init__), let's come up with some way to keep them more declaratively, too.