compiler/gopherjspkg: explicitly mention the vfsgen dependency

The natives package has a blank import of the vfsgen package so that
it stays included in go.mod (otherwise go mod tidy considers it as
unused and removes it). Copy that mechanism here.

Place it under a file with a "generate" build constraint so it doesn't
get included in a normal gopherjs build, since it's only needed during
code generation.

Author: dmitshur

compiler/gopherjspkg/doc.go

@@ -4,10 +4,6 @@
 // operation. They are needed to build the Go standard library with GopherJS.
 // Currently, they include:
 //
-// 	github.com/gopherjs/gopherjs/js
-// 	github.com/gopherjs/gopherjs/nosync
-//
+//	github.com/gopherjs/gopherjs/js
+//	github.com/gopherjs/gopherjs/nosync
 package gopherjspkg
-
-//go:generate vfsgendev -source="github.com/gopherjs/gopherjs/compiler/gopherjspkg".FS -tag=gopherjsdev
-//go:generate gofmt -w -s fs_vfsdata.go

compiler/gopherjspkg/gen.go

@@ -0,0 +1,11 @@
+//go:build generate
+// +build generate
+
+package gopherjspkg
+
+import (
+	_ "github.com/shurcooL/vfsgen" // Force go.mod to require this package.
+)
+
+//go:generate vfsgendev -source="github.com/gopherjs/gopherjs/compiler/gopherjspkg".FS -tag=gopherjsdev
+//go:generate gofmt -w -s fs_vfsdata.go