feat: support `ibis.postgres.connect("postgresql://...")` in addition to `ibis.connect("postgresql://...")`

[NickCrews]

Is your feature request related to a problem?

ibis.connect("postgresql://...") already works, but in my app I started with ibis.postgres.connect("postgresql://...") and this failed, so I started filing this issue before I looked at the code and discovered the workaround.

What is the motivation behind your request?

No response

Describe the solution you'd like

I think this is already implemented in other backends, so just emulate what they do? I could write a PR but I'm not sure how to write the tests. If this is simple for you then awesome, otherwise let me know and I can put in the time to write the PR.

What version of ibis are you running?

main

What backend(s) are you using, if any?

postgres

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