cmdlib: Lower cost of cosa RPM overrides repo

[jlebon]

Surprisingly, AFAICT there is nothing in libdnf today which
automatically lowers the cost of local repos vs remote ones. Let's give
it some help by adding a cost=500 here (the default is 1000). This
will allow libsolv to always pick the RPM from the overrides repo, even
if the same NEVRA is available from a remote repo.

[arithx]

/approve

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[jlebon]

One may wonder, if it's the same RPM from the repos, then why not just let cosa fetch get it from the repos? The answer is twofold:

1. cosa fetch right now doesn't fetch from cosa overrides, both because it's temporary, and also because well, it's already local. Also, one really expects adding RPMs to overrides/rpm then running cosa build directly without re-running cosa fetch to Just Work even though cosa build is fully offline.
2. It may be the same NEVRA, but not actually be the same RPM as the build. This can happen for example, if you did fedpkg local on the pkg you wanted to test but forgot to bump the release. We want the overrides repo to take priority (see [merged] core: Checksum package checksums, not just NEVRA for change detection rpm-ostree#296).

[dustymabe]

for my education.. we are setting the cost=500 here. What is the default cost if it's not set?

nvm - it's in the commit message (default = 1000)

[dustymabe]

/lgtm