* support subrepos
This adds support for get/list/api/import/ls on a subrepo.
Also fixes#3180, by adding a granular url support for `dvc.api.get_url`
And, of course it fixes#3369.
* Add tests
* Fix tests, started to get lock_error
* Fix tests for windows
* try to fix mac builds
* Fix comments, readd removed comments
* Fix test
* address review suggestions:
* use `spy` instead of `wraps`
* remove dvc fixture in a test
* set `cache_types` on Git repo