Automatically add common project roots (src) to search paths

[jakebailey]

Requires #1388. Based on work in #1384.

Once search path configuration is done outside of the initialization options we should:

• Add a new boolean to toggle a search path "heuristic".
• If enabled (and the old backwards-compat searchPaths/extraPaths/etc are not specified), then we should detect a common import root and automatically add it to the search path list.

To start, this can be as simple as looking in the workspace's top level for a src directory, then add to the top of the user search path lists.

In the future, this heuristic can either be expanded, or eliminated (if our import handling becomes loose enough to not require it).