Enhancement: recipe linter: detect depending on itself

[JohannesBuchner]

I was making a conda recipe for one package, then realised a package that it depends on is not yet on conda-forge. Starting with the same recipe, I made the other package. Accidentally, I then put in the requirements the package as its own dependence. docker failed.

I think the linter could already detect this (and maybe even avoid the CO2 cost of the Azure pipeline run).

Putting the package name in package: name: into a subsection of requirements:, for example build, host or run is never correct.

[JohannesBuchner]

This should be labeled enhancement and linter.