Make "default" an alias instead of the canonical driver name.

[SpacemanPaul]

Reason for this pull request

• The canonical name of the postgres driver was "default", with "postgres" and "legacy" as aliases.
• If dynamic driver loading failed, only the "postgres" driver was being forcibly injected.

Proposed changes

• The canonical name of the postgres driver is "postgres", with "default" and "legacy" as aliases.
• If dynamic driver loading fails, the "postgres", "postgis" and "memory" drivers are forcibly injected.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.44%. Comparing base (a5622d8) to head (51695e7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop-1.9 #1590 +/- ## ============================================ Coverage 85.44% 85.44% ============================================ Files 140 140 Lines 15477 15479 +2 ============================================ + Hits 13224 13226 +2 Misses 2253 2253 ```

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