Restructure autogenerated gapic/veneer files into a separate subpackage

[gkevinzheng]

 we discussed the possibility of eventually restructuring the gapic/veneer files into different folders, which should remove the need for this fix, right?

It might be a good idea to open a bug for that and add a link to the comment here

Originally posted by @daniel-sanche in #804 (comment)

[gkevinzheng]

Work will be done on googleapis/google-cloud-python#15410 instead.