- [conan-center] skip hook messages for dependencies

[SSE4]

it's pretty annoying that conan-center hook reports not only errors/warnings for the package being built/tested, but for all of its dependencies. they are obviously useless and uninteresting for the contributor.

it's mainly for the post_package_info hook right now, as other methods besides package_info (export, source, build, package) are only executed for the package being built/tested.

from the signature of the hook, there is no easy way to say if current conanfile/reference is the main one, or dependency:

def post_package_info(output, conanfile, reference, **kwargs):

there is nothing interesting in public ConanFile fields as well.

to complicate things, it seems like the hooks object is being re-created each time, so it's not trivial to save any state between method invocations, it's always brand new, constructed from scratch.

the reliable process-wide storage is sys.modules, which works well, regardless of how many times python file is loaded.

[SSE4]

it's a bit complicated, as we don't invoke conan create in CI, we do conan install --build=<package> and then conan test. so pre_export is not called in the same process as post_package_info. I'll change it.