Allow specification classes relative to (and hence distinct from) their parent module name

[adwait]

In the config.json file, allow specification paths of form:

1. path/file, which is a specification generated from class file in file path/file.py. Here path is a standard "/" separated file path, and file is a string without / or ..
2. path/file.classname, which is a specification from class classname in file path.py. Here path is a standard "/" separated file path.
3. will be deprecated: spec: which is a specification generated from the class spec in file specs/spec.py (in default specs directory)