x/tools/gopls: eagerly compute the workspace symbol index

[findleyr]

This is a spin-off of #52602, which we're going to close as just an optiimization.

In the future, we should go one step further and populate the workspace symbol index at load time. There's a straightforward way to do this: while type-checking we always have access to the full AST of each package, before filtering. If we eagerly compute the symbol table at that time, it should be ~free.

However, I don't want to do this yet, because the cost of inheriting all these symbol handles on each generation is still too high, and our heuristic for which symbol handles to keep around is essentially "all of them", not just those that are reachable from workspace packages.

Once we have more control over snapshot data, we can re-consider this eager calculation.

[findleyr]

I think it might suffice to store pre-computed symbol information on disk, and load from disk rather than parsing and re-building symbol information from scratch. If that takes the initial symbol time for Kubernetes down below ~1 second, no need to pre-build symbol data.

@adonovan: this may be a good proof-of-concept for on-disk indexing.