search: rename 'unindexed' -> 'textsearch' package

Created by: rvantonder

Going to pick textsearch to rename unindexed to (not super attached), but point is to represent shared responsibility for calling out to zoekt/indexed search and searcher/unindexed. After this rename I'm going to factor out searcher-only logic into its own package.

The state will then be:

• internal/search/textsearch/textsearch.go
• internal/search/zoekt/indexed_search.go,...
• internal/search/searcher/...
• internal/search/structural/...

Feel free to share thoughts on this structure. One thing that sticks out is that we can have

internal/search/indexed and internal/search/unindexed

instead of

internal/search/zoekt and internal/search/searcher

I think I prefer the zoekt,searcher pair which makes more sense if we ever want to add other kinds of indexed or unindexed services/backends. This why I'm leaning towards internal/search/searcher package rather than keep unindexed terminology around.

Test plan

Semantics preserving, covered by unit tests.