refactor InMemoryURIResolver

[jurgenvinju]

It would be better if we have one memory:// scheme which hosts for every authority string a different in-memory file-system.

The existing in memory schemes which inherit from the currently abstract InMemory resolver could be rewritten to logical schemes, e.g. test-modules:// would resolve to memory://test-modules/.

The benefit is that more in-memory file systems can be instantiated without adding more code, and also it becomes transparent which schemes are in-memory filesystems (they would all have the memory scheme.

[DavyLandman]

this has been done recently.