Add support for using only one output base

[rockbruno]

The issue with using a separate output base is that we cannot make good use of index while building. This PR then adds an options for everything to happen on a single output base to enable regular build actions to pre-populate the index for us.

WIP: Still trying to understand which particular setup (compile_top_level, etc) results in the strongest gain and whether or not sourcekit-lsp behaves correctly when requests are cancelled.

(On hold: Want to try solving the root issue with output groups first)

[josh-arnold-1]

Would our "index flag" cause our BSP building in single output base to invalidate the cached results?

[luispadron]

It might be good to run the single-output base build with --preemptible so that if a user runs bazel build the BSP build is terminated and the user build is unblocked

[rockbruno]

@josh-arnold-1 Only if you set it up to produce different results. If you see the Example project the flags there don't do anything special so it would work in that situation.

@luispadron Yes, that's already the case today. I am not 100% convinced that sourcekit-lsp can deal with canceled requests correctly, but haven't bumped into big issues so far

sourcekit-bazel-bsp/Sources/SourceKitBazelBSP/RequestHandlers/PrepareHandler.swift

Line 46 in 5a4691b

"--preemptible"

[josh-arnold-1]

@josh-arnold-1 Only if you set it up to produce different results. If you see the Example project the flags there don't do anything special so it would work in that situation.

Thanks! Sorry for dumb question — is there anyway I can check to see if the flags cause invalidation? Would bazel print somethiing when running bazel build, like "discarding analysis" or something?

[rockbruno]

@josh-arnold-1 For analysis yes, the output will mention it. It will also be very slow to switch back and forth between the variants.