Make imports package internal.

It's currently meant to be used by GopherJS Playground only, make that
intent more obvious.

Ideally, the upstream package can be refactored in a way so that it can
be imported and used by environments without access to a local
filesystem, then this internal package can go away.

	gomvpkg -from github.com/gopherjs/gopherjs.github.io/playground/imports -to github.com/gopherjs/gopherjs.github.io/playground/internal/imports

Author: dmitshur

playground/imports/fix.go renamed to playground/internal/imports/fix.go

@@ -10,7 +10,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/gopherjs/gopherjs.github.io/playground/imports"
+	"github.com/gopherjs/gopherjs.github.io/playground/internal/imports"
 	"github.com/gopherjs/gopherjs/compiler"
 	"github.com/gopherjs/gopherjs/js"
 	"github.com/neelance/go-angularjs"