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.