perf: warmup index before the engine starts

[doorgan]

This does a few things:

1. Updates the index to not need the engine at all for indexing. By reading the .beam files first we can build our own small Code Server, which we then use to ask the same questions that we ask the engine for source indexing.
2. If there is no index yet, a best-effort indexing happens using the user's _build folder to build an initial index before the engine even starts. This is specially useful for projects that take a while to compile, as we can immediately be useful to the user while the engine starts and their project gets compiled. I say it is best effort because the user may not have compiled their project yet, or the compiled artifacts are older than latest code changes.

The main thing I have in mind after this is to pave the way for a no-compile mode, where we provide the best support we can with no compilation at all. This change should already provide all the pieces for that.

A secondary motivation is to make the engine as small as we can, to reduce engine preparation time and disk usage.