Add symbols imported in packages to workspace

[lionel-]

Addresses posit-dev/positron#2252.
Addresses posit-dev/positron#8549.
Addresses posit-dev/positron#8550.
Progress towards posit-dev/positron#2321.

Branched from #870. It was rather easy to implement based on the infrastructure provided in that PR.

This fixes diagnostics for imported symbols but I was still seeing some weirdness with local definitions because we didn't synchronise the indexer and the diagnostics properly:

ark/crates/ark/src/lsp/state_handlers.rs

Lines 414 to 417 in 7175d83

	// FIXME: The initial indexer is currently racing against our state notification
	// handlers. The indexer is synchronised through a mutex but we might end up in
	// a weird state. Eventually the index should be moved to WorldState and created
	// on demand with Salsa instrumenting and cancellation.

This is now fixed. I've also made a change to take into account objects assigned globally. We were detecting global functions but not other kinds of objects.

I've hacked in testthat imports inside testthat/ files. Should be good enough for now. Will fail when people edit their testthat.R file with additional library loading.

QA Notes

You should now be able to open a package like ellmer and not see diagnostics. This won't be 100% proof for all packages, but I've checked with rlang and ellmer.

See also posit-dev/positron#8549 and posit-dev/positron#8550 for reprexes for adjacent fixes.

[DavisVaughan]

Reconsider batching? Based on zoom call

[lionel-]

I take it back, the interleaved indexer/diagnostics processing will arise only if the queue tasks are processed faster than they arrive so the current setup is fine. If we get a bunch of tasks very rapidly, we will collect them, split them by type, and process the indexer tasks first. So the batching is still useful.

This setup will be entirely replaced by Salsa dependencies. Diagnostics tasks will be cancelled automatically as document updates arrive. So we shouldn't worry about this temporary setup too much.

[lionel-]

I adapted the loop so that we check back for more indexer tasks once we have finished a round of indexing. We do that at most 10 times so diagnostics get refreshed once in a while if the user is writing too fast to keep up. I think I'm happy with the queue setup now!

[lionel-]

@DavisVaughan I'm out of the day but this should be ready for review. When I come back I'll add some diagnostics tests for packages.