Handle on-the-fly search path configuration changes, read search paths from config directly

[jakebailey]

Fixes #1388.
Fixes #915. (Null check added.)
Updates #469. (Less needed from initializationOptions, PYTHONPATH potential.)

User configured search paths are now pulled from the user configuration (and on the fly), rather than solely through the initialization options.

To do this, the LS initialization logic has been moved from initialize RPC call into the initialized notification handler. There, we can ask the editor for the python configuration, and then use the real configuration to set the initial search paths. If the configuration is changed later, then it will be re-processed, and a reload will be triggered if it changes (just like path watching).

The old python.autoComplete.extraPaths will still work. python.analysis.searchPaths and python.analysis.usePYTHONPATH have been added as a newer interface. Also in this change is a fix which makes watchSearchPaths accessible again, as python.analysis.watchSearchPaths; previously it was not accessible.

Config items:

• searchPaths in the initialization options - Old, last resort.
• python.autoComplete.extraPaths - Old, but still read.
• python.analysis.searchPaths - New, replacement for extraPaths. If present, extraPaths is ignored entirely.
• python.analysis.usePYTHONPATH - New, true (default) if PYTHONPATH should be added to the end of searchPaths/extraPaths (which the extension did)
• python.analysis.watchSearchPaths - Fixed, true (default) if library search paths should be watched for file changes.

[jakebailey]

I can see a way to handle the issues with the configuration coming in too late: The server can make a workspace/configuration call back to the editor and ask for the configuration. However, the server is not allowed to make any calls until initialize returns, which means that any initialization RPC calls back to the client need to happen later, which the spec says to do in the initialized handler. It's very likely that what we're doing in the initialize handler can be moved later.

That might be too invasive to do in this change, but, if it's done, the server can just ask the editor for the config, pre-populate the settings (before it ever gets workspace/didChangeConfiguration), and initialize based on that.