[feature] conditional alias packages

[KerstinKeller]

I would like to be able to make conditional aliases. I would like to use them to model system versions, and require them like:

from conans import ConanFile

class ConsumerConanfile(ConanFile):
    requires = "protobuf/(system)@"

With the current definition, I cannot make aliases depend on settings.

It would be cool to have a set_alias() function (similar to set_name(), set_version() functions).
Or to be able to set the values in configure() instead (which actually also works for the version attribute, instead of using set_version())

from conans import ConanFile

class ProtobufSystemConanfile(ConanFile):
    
    def set_alias(self):
        if self.settings.os.distro = Ubuntu16:
            self.alias = "protobuf/3.0.0@heat-rae/stable"
        else self.settings.os.distro = Ubuntu18:
            self.alias = "protobuf/3.6.1@heat-rae/stable"
        else:
           # throw invalidConfigurationException

• I've read the CONTRIBUTING guide.

[memsharded]

This seems to be a duplicate of #9510

[KerstinKeller]

Ah ok, I'll close this in favor of the other issue.