URL can in fact store URIs, it just doesn't have a very nice API to
interact with them. As long as we only operate on absoluteString, we
should be fine though. So instead of implementing the logic for
detecting file URLs ourselves, we can just use a URL as storage for
DocumentURI.
According to the LSP specification, arbitrary URIs can be used as
document identifiers. Instead of internally assuming that all URIs are
URLs, use a DocumentURI enum to represent URIs. These can either be file
URLs or other URIs whose value as treated as an opaque string.
We will be able to split the LSP modules off later. These LSP modules
will provide the ability to write custom LSP servers and clients in
Swift. The sourcekit-lsp repository will build on top of this new
package to provide an LSP implementation that creates a language server
for Swift and C-based-languages.
Introduce a `BuildSystemDelegate` to handle notifications from the build system
* `SourceKitServer` is the main delegate to process these notifications
* Currently limited to changes in `FileBuildSettings`
* Delegate informs the `BuildSystem` of files to watch via `registerChangeWatching(for: URL)` and `unregisterChangeWatching(for: URL)`
* In the future we could have more integration for handling changes in dependencies
Handling changes in `FileBuildSettings`
* `SourceKitServer` sends notifications to the internal LSPs informing them of any opened documents that have changes in their compiler flags
* For clangd, we send a notification to update the compilation database
* For SourceKit/sourcekitd we must close and reopen the file to force a new AST with the new compiler flags
