Search: fold unindexed search into Zoekt

[jtibshirani]

It could be really nice if Zoekt handled unindexed search instead of our separate 'searcher' service. Some benefits:

• Unindexed searches would benefit from Zoekt's ranking
• We wouldn't to have to implement new search features in multiple places
• It'd simplify our "hybrid search" implementation (and make it more efficient?)

This feels like a large and fairly tricky project, since Zoekt is not factored in such a way that makes it easy to load and parse content on the fly. But it'd save us time in the longer term, as we'd no longer need to implement search features in two separate places. Some recent examples of this extra work:

• https://github.com/sourcegraph/sourcegraph/issues/59038
• https://github.com/sourcegraph/sourcegraph/issues/60341

/cc @sourcegraph/search-platform

[keegancsmith]

Not sure how useful it is, but I got started on factoring out the matchtree abstraction into a public API like 6 years ago :) I think my plan then was to use it inside of searcher so we could support and/or queries efficiently, but at the time we decided against it at the time so I reverted those changes. https://github.com/sourcegraph/zoekt/blob/dacbbfd8c3ceab2bb3288d28ce3f35d3a3eb3c13/matchtree/matchtree.go