Automatic user/channel in <host_version>

[AbrilRBS]

Changelog: TODO
Docs: TODO

Started as a test for replace requires with host_version chains when the user/channel didn't match, see
#19598

[memsharded]

I think this looks good and could be moved forward

[franramirez688]

LGTM!
Only a small comment about this case:

                def requirements(self):
                    self.requires("protobuf/1.0@user/channel")
                def build_requirements(self):
                    self.tool_requires("protobuf/<host_version>@user/channel")

Could we raise an error and say something like It's not allowed to use the "@user/channel" when "<host_version>" is declared??