Address some `kart export` review comments - part 2

[olsen232]

Mostly:

• CRS transforms
• Making sure the user can't overwrite layers without trying to
• Making sure multiple layer datasets / directories work where expected
• Various tests

Related links:

https://github.com/koordinates/kart/pull/988 contains comments recommending changes along these lines.

Checklist:

• [x] Have you reviewed your own change?
• [x] Have you included test(s)?
• [ ] Have you updated the changelog?