warn if private imports from custom commands

[memsharded]

Changelog: Omit
Docs: https://github.com/conan-io/docs/pull/XXXX

The output will be a summary:

WARN: ********************************************************************************
WARN: Forbidden internal import'from conan.internal' in:
WARN:     T:\tmp7dfhixvmconans\path with spaces\.conan2\extensions\commands\cmd_mycache.py
WARN: Forbidden internal import'from conans' in:
WARN:     T:\tmp7dfhixvmconans\path with spaces\.conan2\extensions\commands\cmd_mycache.py
WARN: 
WARN: These imports will change in future releases
WARN: Please, use only public documented API in docs.conan.io
WARN: ********************************************************************************
DATA_YML: conandata.yml

[memsharded]

Dropping this, it will be problematic:

• It only applies to custom commands, not other usages of Conan API
• It cannot be silenced with core:skip_warnings because the loading of imports happen earlier than processing the arguments
• For same reason, it cannot be made warn-as-errors
• Users already using imports, or accepting they might be broken will not have a way to silence the warnings, and will be annoying

Unless a better way that can be silenced, and hopefully more generic, this seems not worth the complexity and risk.