Allow passing sys_path to Jedi from the settings

[Kronuz]

Additionally to source_roots, it should be possible to pass additional sys_path from the settings. Issues arise when using Python2/Python3 codebases and simply setting PYTHONPATH.

[Kronuz]

The main issue with not being able to do this is one has to recur to the method of modifying PYTHONPATH environment variable... but this breaks when Python2 and Python3 are both installed and the included PYTHONPATH has incompatible Python2 vs Python3 packages.

For example, when setting PYTHONPATH to a path that contains Python2's enum, and having Python3 installed, Jedi some times tries to evaluate code using python3, but the PYTHONPATH the evaluator process is called with is that one with enum in it (or some other Python2 specific packages), so Python3 breaks with the error:

AttributeError: module 'enum' has no attribute 'IntFlag'

[drmathochist]

I came here with a similar question, but I have a use case where the extra source roots to use may depend on the Document path itself, and not be available at LSP initialization time.

My thought was that we might make source_roots itself "pluggable", and factor out the current implementation to a built-in plugin matching the hook signature

@hookspec
def source_roots(doc_path, root_path):
    pass

Users can then implement their own add-ons, whether constant or depending on the document or root path, as plugins. How does that sound?