Config validation fixups

[kolyshkin]

With this fix, the config validation now properly detects the dupe that was fixed earlier in commit a8c7f00 / PR #106 (which it failed to detect before).

With that commit reverted, the failure looks like this:

=== RUN TestConfigs config_test.go:47: main+4.13 combined config failed validation: errors: ; warnings: config entry [[rpm.containernetworking-plugins.ignore]].error=ErrGoMissingTag.files contains a redundant path "/usr/libexec/cni/dummy", overlapped by "/usr/libexec/cni" --- FAIL: TestConfigs (0.00s)

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kolyshkin

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[openshift-ci[bot]]

@kolyshkin: all tests passed!

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[rphillips]

/lgtm