internal/lsp: don't search for workspace modules by default

This may be expensive, and we don't want to do it if the results will
be unused.

Updates golang/go#41558

Change-Id: I0c01a80f4fc459e1cc189934ad23ab443e5ea2a5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/257137
Trust: Rebecca Stambler <rstambler@golang.org>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/lsp/cache/session.go

@@ -166,10 +166,14 @@ func (s *Session) createView(ctx context.Context, name string, folder span.URI,
 		return nil, nil, func() {}, err
 	}
 
-	// Find all of the modules in the workspace.
-	modules, err := findWorkspaceModules(ctx, ws.rootURI, options)
-	if err != nil {
-		return nil, nil, func() {}, err
+	// If workspace module mode is enabled, find all of the modules in the
+	// workspace.
+	var modules map[span.URI]*moduleRoot
+	if options.ExperimentalWorkspaceModule {
+		modules, err = findWorkspaceModules(ctx, ws.rootURI, options)
+		if err != nil {
+			return nil, nil, func() {}, err
+		}
 	}
 
 	// Now that we have set all required fields,