Bug 1879168

Description of problem:
The dnf documentation [1] says that the dnf.package.Package.reponame attribute is the "Id of repository the package was installed from (string)." Unfortunately, this attribute is always equal to "@System", no matter the repository a package has been installed from.

[1] https://dnf.readthedocs.io/en/latest/api_package.html?highlight=package#dnf.package.Package.reponame

Version-Release number of selected component (if applicable):
dnf-0:4.2.17-7.el8_2.noarch
rpm-0:4.14.2-37.el8.x86_64

Steps to Reproduce:
Run this piece of python code on RHEL 8.2:
```
import dnf
b = dnf.Base()
b.fill_sack(load_system_repo=True, load_available_repos=False)
for pkg in b.sack.query():
    print(pkg.name + '-' + pkg.evr + ': ' +  pkg.reponame)
```

Actual results:
The name of the repository from which a package has been installed is always @System:
...
NetworkManager-tui-1:1.22.8-5.el8_2: @System
acl-2.2.53-1.el8: @System
audit-3.0-0.17.20191104git1c2f876.el8: @System
...

Expected results:
I expect the reponame to match the repoid from which each package has been installed, e.g. "@rhel-8-for-x86_64-appstream-rpms".

Additional info:
The undocumented attribute dnf.package.Package._from_repo contains the correct repoid. When I use it instead of the reponame attribute, I get:
...
NetworkManager-tui-1:1.22.8-5.el8_2: @rhel-8-for-x86_64-baseos-rpms
acl-2.2.53-1.el8: @rhel-8-for-x86_64-baseos-rpms
audit-3.0-0.17.20191104git1c2f876.el8: @rhel-8-for-x86_64-baseos-rpms
...