gopls/internal/cache: check views on any on-disk change to go.mod files

To prepare for the follow up change, where we honor local replaces in
the view selection algorithm, treat go.mod files more like go.work files
in a couple places:

- Re-run the view selection algorithm whenever a go.mod file changes on
  disk.
- Use the workspaceModFiles field in the bestView algorithm, rather than
  checking the gomod field directly.

Apart from perhaps running the view selection algorithm more frequently,
this change should have no observable impact.

For golang/go#64888

Change-Id: I9d9a74b876791a77e284a1a1d5fcc7a691199901
Reviewed-on: https://go-review.googlesource.com/c/tools/+/562679
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: findleyr

gopls/internal/cache/session.go

@@ -590,7 +590,7 @@ func bestView[V viewDefiner](ctx context.Context, fs file.Source, fh file.Handle
 				pushView(&workViews, view)
 			}
 		case GoModView:
-			if modURI == def.gomod {
+			if _, ok := def.workspaceModFiles[modURI]; ok {
 				modViews = append(modViews, view)
 			}
 		case GOPATHView:
@@ -746,18 +746,12 @@ func (s *Session) DidModifyFiles(ctx context.Context, modifications []file.Modif
 			checkViews = true
 		}
 
-		// Any on-disk change to a go.work file causes recomputing views.
+		// Any on-disk change to a go.work or go.mod file causes recomputing views.
 		//
 		// TODO(rfindley): go.work files need not be named "go.work" -- we need to
 		// check each view's source to handle the case of an explicit GOWORK value.
 		// Write a test that fails, and fix this.
-		if isGoWork(c.URI) && (c.Action == file.Save || c.OnDisk) {
-			checkViews = true
-		}
-		// Opening/Close/Create/Delete of go.mod files all trigger
-		// re-evaluation of Views. Changes do not as they can't affect the set of
-		// Views.
-		if isGoMod(c.URI) && c.Action != file.Change && c.Action != file.Save {
+		if (isGoWork(c.URI) || isGoMod(c.URI)) && (c.Action == file.Save || c.OnDisk) {
 			checkViews = true
 		}