I’d like to qualify `--experimental-prepare-for-indexing` independently of background indexing using `swift build`. Because of this, I think there is value in using SourceKit-LSP using background indexing but without `--experimental-prepare-for-indexing`. It could also be useful to determine if bugs we may find are due to `--experimental-prepare-for-indexing` or also occur when running plain `swift build` commands.
Background indexing probably won’t be the last experimental feature in sourcekit-lsp that we want to gate behind a feature flag. Instead of adding new parameters ad-hoc, introduce a general notion of experimental features.
When looking for a workspace that can handle a file, we were creating full-fledged workspaces along the way, which we would then discard if they couldn’t handle the file being opened. This had multiple problems:
1. When background indexing is enabled, it caused semantic indexing of the workspace, which wrote files to a `.index-build` directory and was a waste of work
2. When background indexing is enabled, it caused package resolution, which also created a `.index-build` folder to be created
3. It caused a syntactic test index of the workspace, which was a waste of work.
To fix this, do multiple things:
1. When creating a workspace, add a check right after build system creation. This allows us to early exit if the build system can’t handle the file and prevents us from generating the `Workspace`, fixing (1) and (3)
2. Don’t call `reloadPackage` when creating a `SwiftPMWorkspace`. Instead, explicitly call `generateBuildGraph` once we committed to creating the workspace.
