Lazy loading follow-up items

With support for lazy loading of assemblies, we still unfortunately need to scan all referenced assemblies for a few reasons. We should add the following perf optimizations:
- [ ] Avoid resolving all references for '*' in attribute XML (used in corelib, and allowed in command-line XML)
- [ ] We should also avoid an extra lookup in the attribute cache when an assembly has no embedded XML (see https://github.com/mono/linker/pull/1666#discussion_r552239646) - or, change it to a two-level cache (by assembly and attribute provider).
- [ ] Avoid resolving all references when searching for unqualified type names
- In some cases we may be able to match the GetType behavior which looks in the calling assembly and corelib only.