fix vettool; run vet on all available packages (#10389)

#### Summary
Unfortunately, `which shadow` didn't resolve to the shadow binary, so hard-code the expected path in `$GOPATH/bin`. At the same time, run `go vet` across both the server and enterprise (if present), reducing the number of required invocations.

This is accompanied by an enterprise change to fix shadowing issues there.

#### Ticket Link
N/A (tooling upgrade)

#### Checklist
- [x] Has enterprise changes: https://github.com/mattermost/enterprise/pull/403

app/command_test.go

@@ -343,15 +343,14 @@ func TestDoCommandRequest(t *testing.T) {
 	})
 
 	t.Run("with a slow response", func(t *testing.T) {
-		timeout := 100 * time.Millisecond
-
+		done := make(chan bool)
 		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-			time.Sleep(timeout + time.Millisecond)
+			<-done
 			io.Copy(w, strings.NewReader(`{"text": "Hello, World!"}`))
 		}))
 		defer server.Close()
 
-		th.App.HTTPService.(*httpservice.HTTPServiceImpl).RequestTimeout = timeout
+		th.App.HTTPService.(*httpservice.HTTPServiceImpl).RequestTimeout = 100 * time.Millisecond
 		defer func() {
 			th.App.HTTPService.(*httpservice.HTTPServiceImpl).RequestTimeout = httpservice.RequestTimeout
 		}()
@@ -359,5 +358,6 @@ func TestDoCommandRequest(t *testing.T) {
 		_, _, err := th.App.doCommandRequest(&model.Command{URL: server.URL}, url.Values{})
 		require.NotNil(t, err)
 		require.Equal(t, "api.command.execute_command.failed.app_error", err.Id)
+		close(done)
 	})
 }