Use packit for building rpms

[LecrisUT]

Packit recently added the resuldir for building with mock which was a partial blocker for switching the makefile to using it. The checking if mock cli actually works proved a bit more complicated ^1, but would it be a blocker?

To discusss:

• For srpm build, do we let packit decide on the srpm name, which would make it like: fmf-1.6.1.dev1+g1c5d676-1.20250108095718984812.makefile.1.g1c5d676.fc41.src.rpm
• For packit build in-mock where should the resultdir point to?
• Do we make the mock root configurable with Release/arch variables?
• .gitignore entries