Cleanups in `TypeInformation`

[odrotbohm]

• Unify test cases in TypeInformationUnitTests (currently spread around in TypeDiscovererUnitTests and ClassTypeInformation).
• Re-enable caching of raw ResolvableTypes in TypeDiscoverer once Issue with equals(…)/hashCode() for ResolvableType.forRawClass(…) spring-framework#28608 is resolved
• Re-structure type hierarchy TI <-> ResolvableTI <-> CTI, GTI to simplify equals(…) / hashCode() arrangements by moving all methods except the ones overridden in CTI into RTI. Current implementations of those methods in TypeDiscoverer into GTI.