gopls/internal/analysis/modernize/cmd/modernize: create

By moving the main.go file, gopls users will be able to run

$ go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix

after the gopls release to apply all modernizer fixes en masse.

Updates golang/go#70815

Change-Id: I25352b7b77653c177310dfea7a050741949890f9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/647355
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

gopls/internal/analysis/modernize/main.go → gopls/internal/analysis/modernize/cmd/modernize/main.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build ignore
-
 // The modernize command suggests (or, with -fix, applies) fixes that
 // clarify Go code by using more modern features.
 package main