Add support for a path-based MetadataAssemblyResolver

[steveharter]

As part of issue https://github.com/dotnet/corefx/issues/2800 and PR dotnet/corefx#33201 we would like a default path-based resolver than can work with standard build-related scenarios.

We need requirements for how duplicate assembly references are addressed. See the discussion in the PR pertaining to PathAssemblyResolver.

[steveharter]

Based on discussion, the proposal is to:

• Assume a filename:simplename mapping.
  • This allows "lazy load" of the assemblies specified by the paths, and aligns with other semantics in the CLR.
• Duplicate filenames (in different directories) are resolved by picking the highest Version per PublicKeyToken.
  • If there is no PublicKeyToken for an Assembly, it is treated as a "blank" PublicKeyToken.

In addition, we need to change MetadataLoadContext to no longer "cache" assemblies that are not returned from Resolve(). This allows PathAssemblyResolver and other resolvers control over mapping of Version and other properties - for example, allow a resolver to return the highest Version (e.g. 2.0.0.0) even if the request for a Version (e.g. 1.0.0.0) matches a previously loaded assembly exactly.