Currently the "path" module does two very different things:

* Defines how ipfs paths look like and provides tools to parse/split etc.
* Provides a resolver to resolve paths.

This moves the resolver stuff to `path/resolver` and leaves the
path utilities in `path`.

The result is that now the IPFS `path` package just defines what a path
looks like and becomes a module that can be exported/re-used without problems.
Currently there are circular dependency cycles (resolve_test -> merkledag/utils,
merkledag->path), which the prevent the export of merkledag itself.

License: MIT
Signed-off-by: Hector Sanjuan <hector@protocol.ai>