[BSP] Use consistent file URI format (#2803)

Previously we were emitting different URI formats if the sourceFile info
was missing or not.
Not sure if this is a problem, but `targetId.getUri` is returning files
as `file:///foo/bar/file.scala` while our `java.io.File` `toURI` returns
`file:/foo/bar/file.scala`
Changing it to `.toPath.toUri` for consistency

Pull request: #2803

Co-authored-by: Tobias Roeser <le.petit.fou@web.de>

bsp/worker/src/mill/bsp/worker/BspCompileProblemReporter.scala

@@ -69,9 +69,11 @@ private class BspCompileProblemReporter(
     val diagnostic = getSingleDiagnostic(problem)
     val sourceFile = problem.position.sourceFile
     val textDocument = new TextDocumentIdentifier(
-      sourceFile.getOrElse(None) match {
+      sourceFile match {
         case None => targetId.getUri
-        case f: File => f.toURI.toString
+        case Some(f) =>
+          // The extra step invoking `toPath` results in a nicer URI starting with `file:///`
+          f.toPath.toUri.toString
       }
     )
     val params = new bsp.PublishDiagnosticsParams(