basic-code-intel uses version contexts

Created by: keegancsmith

Part of RFC 136: Navigating version contexts

basic-code-intel uses search to find definitions and references. When a version context is active it should use it to prioritize results from it. This requires adding a way to pass in version context to basic-code-intel. This then needs to be threaded through to search calls. A fallback to default context is probably also needed. How basic-code-intel currently uses search (thanks @efritz):

For search-based definitions we do a symbol search at repo:current_repo@current_commit and fallback to repo:current_repo index:yes for local defs, then do a repo:-current_repo index:yes for remote definition fallbacks.

For search-based references we do a text search with the same basic shape (except we do them concurrently).