This PR fixes a number of bugs related to the handling of the source search path in the language server, where deleting files could cause several features to stop functioning and both untitled files and files that don't exist on disc could have conflicting module names.

In detail, it makes the following adjustments:

• The URI <-> module name conversion was adjusted to produce no name collisions.
  • File URIs in the search path yield a module name relative to the search path, as before.
  • File URIs not in the search path, non-file URIs and non-.lean files yield a «external:<full uri>» module name.
• To avoid the issue of the URI -> module name conversion failing when a file is deleted from disc, we now cache the result of this conversion in the watchdog and the file worker when the file is first opened.
• All of the URI <-> module name conversions now consistently go through Server.documentUriFromModule? and moduleFromDocumentUri to ensure that we don't have minor deviations for this conversion all over the place.
• The threading of the source search path through the file worker (from lake setup-file) is removed. It turns out that lake serve already sets the correct source search path in the environment, so we can just always use the search path from the environment.
• Since we can now answer more requests that need the .ileans in untitled files, a lot of the tests that test 'Go to definition' needed to be adjusted so that they use the information from the watchdog, not the file worker. As we load references asynchronously, this PR adds an internal $/lean/waitForILeans request that tests can use to wait for all .ilean files to be loaded and for the ilean references from the file worker for the current document version to be finalized.
• As part of this PR, we noticed that the .ileans aren't available in the NixOS setup, so @Kha adjusted the Nix CI to fix this.

Breaking changes

• Server.documentUriFromModule has been renamed to Server.documentUriFromModule? and doesn't take a SearchPath argument anymore, as the SearchPath is now computed from the LEAN_SRC_PATH environment variable. It has also been moved from Lean.Server.GoTo to Lean.Server.Utils.
• Server.moduleFromDocumentUri does not take a SearchPath argument anymore and won't return an Option anymore. It has also been moved from Lean.Server.GoTo to Lean.Server.Utils.
• The System.SearchPath.searchModuleNameOfUri function has been removed. It is recommended to use Server.moduleFromDocumentUri instead.
• The initSrcSearchPath function has been renamed to getSrcSearchPath and has been moved from Lean.Util.Paths to Lean.Util.Path. It also doesn't need to take a pkgSearchPath argument anymore.