Expose conanfile.output to ConanFileInterface

[AbrilRBS]

Changelog: Omit
Docs: Omit

For conan-io/examples2#61, the copy method in tools.files expects a ConanFile as its first argument, but there's not much stopping us form also using a ConanFileInterface, so that for example copy can be called on files from self.depenencies. The only issue was a missing exposed property, so this adds that to the interface.

This might not be a good solution as it expands the public surface of this file, and there's a possible workaround in passing None as the argument which suppresses all output, so I'm happy to close if desired :)

[memsharded]

Hi @RubenRBS

The idea is that the output of the dependency shouldn't be used from the consumers (self.dependencies) side. All it does is to encapsulate the package prefix (like pkg/version: ) to the output, but this isn't what should be shown in output, but the one of the consumer side calling it. The intention of that prefix is that messages outputed from the recipe have that prefix, but messages from the consumers have the prefix of the consumer. In this case, the one doing the copy() is the one to be passed, not the dependency.

[AbrilRBS]

Oh, that's completely reasonable, thanks for your input, much appreciated :)