Rework 'type decimal64' lookup

SourceSpecificContext contains a hack, attempts to lookup specific complex types by considering only a QName's localName and routes it via BUILTIN_TYPE_SUPPORTS to specific statement support.

This may produce wrong results on wrong use, for example if one were to do:

module foo {
extension foo;
}

module bar {
foo:decimal64;
}

Note how bar's use of extension should fail – but it will instead being routed to Decimal64SpecificationImpl.Definition.

Identify the caller, which is probably to a 'type decimal64' or similar construct and is attempting to create a declared statement for it. This needs to be resolved in a way which does not go through SourceSpecificContext, if at all possible.