Address review comments

- Move AppEngineTokenSource documentation to one location
- Add file comments
- Log the deprecation message exactly once in the second generation and
  flex AppEngineTokenSource.

google/appengine_gen1.go

@@ -4,6 +4,8 @@
 
 // +build appengine
 
+// This file applies to App Engine first generation runtimes (<= Go 1.9).
+
 package google
 
 import (
@@ -21,23 +23,8 @@ func init() {
 	appengineAppIDFunc = appengine.AppID
 }
 
-// AppEngineTokenSource returns a token source that fetches tokens from either
-// the current application's service account or from the metadata server,
-// depending on the App Engine environment. See below for environment-specific
-// details. If you are implementing a 3-legged OAuth 2.0 flow on App Engine that
-// involves user accounts, see oauth2.Config instead.
-//
-// First generation App Engine runtimes (<= Go 1.9):
-// AppEngineTokenSource returns a token source that fetches tokens issued to the
-// current App Engine application's service account. The provided context must have
-// come from appengine.NewContext.
-//
-// Second generation App Engine runtimes (>= Go 1.11) and App Engine flexible:
-// AppEngineTokenSource is DEPRECATED on second generation runtimes and on the
-// flexible environment. It delegates to ComputeTokenSource, and the provided
-// context and scopes are not used. Please use DefaultTokenSource (or ComputeTokenSource,
-// which DefaultTokenSource will use in this case) instead.
-func AppEngineTokenSource(ctx context.Context, scope ...string) oauth2.TokenSource {
+// See comment on AppEngineTokenSource in appengine.go.
+func getAppEngineTokenSource(ctx context.Context, scope ...string) oauth2.TokenSource {
 	scopes := append([]string{}, scope...)
 	sort.Strings(scopes)
 	return &appEngineTokenSource{