Test monorepo config

[lachmanfrantisek]

Follow-up to packit/packit-service#1897

Use and try the new monorepo format in Packit config in:

• one of our packages: Use packages syntax for packit itself for dogfooding packit#1969
• hello-world test case: Basic test case: monorepos hello-world#1557
• mock: Use monorepo syntax for Packit builds rpm-software-management/mock#1144
• copr: Use monorepo setup for Packit fedora-copr/copr#2720

If you discover any bugs, fix the easier ones and create follow-up issues for the more complicated ones.

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Part of packit/packit#1543 epic.

[lachmanfrantisek]

After some testing, we need to fix either packit/packit-service#2055 (ideally) or packit/packit-service#2058.