* refactor postgres,
* adds a basic unit test module
* first step towards a common utils module
* set postgresql_db doc argument defaults to what the code actually uses
* unit tests that actually test a missing/found psycopg2, no dependency needed
* add doc fragments, use common args, ansible2ify the imports
* update dict
* add AnsibleModule import
* mv AnsibleModule import to correct file
* restore some database utils we need
* rm some more duplicated pg doc fragments
* change ssl_mode from disable to prefer, add update docs
* use LibraryError pattern for import verification
per comments on #21435. basically LibraryError and touching up its usage in pg_db and the tests.
