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You neglected to add the default CentOS 7.x repos which is likely why you're running into your issue. Incidentally, you can fix this issue by grabbing yet another RPM:

$ rpm -ivh http://mirror.centos.org/centos/7/os/x86_64/Packages/centos-release-7-5.1804.el7.centos.x86_64.rpm

NOTE: You can find out what RPM to acquire using rpm to query to see what RPM provides a given file. In our case these files:

$ rpm -qf /etc/yum.repos.d/CentOS-* | sort -u
centos-release-7-5.1804.1.el7.centos.x86_64

Once we re-install this RPM, it'll refresh our .repo files for the base repositories under /etc/yum.d. If we didn't know the URL where to download it, again, we can inquire to RPM to find out.

First take note of the name of the RPM:

$ rpm -qi centos-release-7-5.1804.1.el7.centos.x86_64 | grep Name
Name        : centos-release

Then search on pkgs.org for this shortname, which will lead you to the URL I showed above in the rpm -ivh ... command.

NOTE: The same steps can be applied for CentOS 7.3, or whatever version of CentOS you're currently using.