Various nits, fixes and cleanup

[ntnn]

Summary

1. Ensure make recipes work across all modules
2. Fix sdk/client importing sdk/apis by merging into one package sdk
   This causes errors when the APIs change but are not present in main, because the generated client tries to import from sdk/apis which upstream isn't updated yet ofc.
3. Fix various fix golangci-lint found (which were hidden because lint only ran on the root module)
4. Fix code generator by updating kcp-dev/code-generator (not sure why but v2.3.1 wouldn't build due to a dependency erroring; v2.4.0 works though)
5. Tidy all modules
6. Also fix the make help output

This was actually intended to be a smaller PR with just the recipe changes but you know about the 99 bugs

What Type of PR Is This?

/kind bug
/kind cleanup

Related Issue(s)

Fixes #

Release Notes

Merge `sdk/client` and `sdk/apis` into `sdk` package

[mjudeikis]

Normally I would say - let's not split sdk/{client,api}
But as we are working towards converting all code to be multicluster-runtime, and hence dont need generated client anymore - sdk/client should go away very soon.
So this makes code simplier for now, and easier to consume.

[mjudeikis]

lets ship this.