Description of problem:
In cases where Red Hat Enterprise Linux 8 chroots are being made from scratch and modular content will be used as part of the initial install, DNF fails to resolve modular dependencies because it cannot discover the modular platform.

DNF does modular platform discovery in two ways:

* Reading the PLATFORM_ID from os-release(5)
* Reading the package providing system-release for base-module() Provides

The former method works on an existing RHEL 8 environment, but when trying to produce a brand new one without an existing reference or manual setting of the value, DNF relies on the latter method.

This declaration does not exist, which causes tools like appliance-creator, livecd-creator, kiwi, and others to fail to be able to resolve modular dependencies, essentially blocking the usage of modules with these tools.
 

Version-Release number of selected component (if applicable):
8.1-3.3.el8

How reproducible:
Always

Steps to Reproduce:
1. Create a directory with .repo files with RHEL repos on a Fedora system
2. dnf --setopt=reposdir=/path/to/rhelreposdir --installroot=/tmp/rhel8root install @core python36

Actual results:
It fails with dependency errors because the platform is not set.

Expected results:
It installs the packages and creates the chroot successfully.

Additional info:
I've fixed this in centos-release with the following change: https://git.centos.org/rpms/centos-release/c/89457ca3bf36c7c29d47c5d573a819dd7ee054fe?branch=c8