extension/src: skip tool existence check if replaced by gopls

When gopls is running, format tools it replaces are ineffective.

This change prevents VS Code Go from prompting for their
installation when gopls is active.

For #3677

Change-Id: Ic1fabf0687b6c9b3687dbc59e2099859b90d2250
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/648415
kokoro-CI: kokoro <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Madeline Kalil <[email protected]>

extension/src/goMain.ts

@@ -75,6 +75,7 @@ import { toggleVulncheckCommandFactory } from './goVulncheck';
 import { GoTaskProvider } from './goTaskProvider';
 import { setTelemetryEnvVars, telemetryReporter } from './goTelemetry';
 import { experiments } from './experimental';
+import { allToolsInformation } from './goToolsInformation';
 
 const goCtx: GoExtensionContext = {};
 
@@ -300,7 +301,16 @@ function addOnDidChangeConfigListeners(ctx: vscode.ExtensionContext) {
 			}
 
 			if (e.affectsConfiguration('go.formatTool')) {
-				checkToolExists(getFormatTool(updatedGoConfig));
+				const tool = getFormatTool(updatedGoConfig);
+				// When language server gopls is active (by default), existence
+				// checks are skipped only for tools that gopls replaces. Other
+				// tools are still checked.
+				if (
+					updatedGoConfig['useLanguageServer'] === false ||
+					!new Set(['gofmt', 'goimports', 'goformat']).has(tool)
+				) {
+					checkToolExists(tool);
+				}
 			}
 			if (e.affectsConfiguration('go.lintTool')) {
 				checkToolExists(updatedGoConfig['lintTool']);