Add import-linter

[coretl]

To avoid making circular imports it would be good to make some rules and add a linter as per DiamondLightSource/mx-bluesky#568 but with the config in a TOML file.

The rules:

• epics depends only on core
• tango depends only on core
• sim depends only on core
• fastcs depends only on core, epics, tango
• plan_stubs can depend on anything

Acceptance Criteria

• Anything that breaks the above rules errors pre-commit

[stan-dot]

I assume that we want core to be independent now.

should plan_stubs be generic though? separate tango and epics plan stubs?

[coretl]

should plan_stubs be generic though? separate tango and epics plan stubs?

Hmm, interesting point, in fastcs we do try/catch imports of epics and tango, and epics works even without the relevant transport, but tango doesn't. This will work at the moment but we might find this breaks in the future.

@abbiemery thoughts?