Replaced some direct usages of IdentityFileResolver with factory methods

- GlobalScopeServices creates a default FileResolver instance.
- Use FileResolver.withNoBaseDir() and withBaseDir() to create new resolvers.
- Use TestFiles fixture to create resolvers in tests.
Moved ClasspathUtil and ClassLoaderFactory and impl to baseServices.
Moved some inner classes from PayloadSerializer to become top level classes
Added a super class for a couple of specs to share fixtures.