refactor: remove ~nelsam packages

[janishorsts]

Mock generators like helheim add significant noise, fragile channel-based synchronization, and unnecessary third-party dependencies to test packages. Hand-written fakes that satisfy minimal interfaces are clearer, faster, and much easier to read and debug.
This change removes git.sr.ht/~nelsam/hel and github.com/nelsam/correct along with all generated mock code.

Changes

• Removed generated mocks: Deleted helheim_mocks_test.go and associated generation helpers in util/proj and util/llbutil/authprovider.
• Simplified test suites: Replaced mock expectations (pers.MethodWasCalled, complex channel assertions) in multiauthprovider_test.go, podman_test.go, and golang_test.go with minimal, explicit test fakes (such as fakeFS, fakeExecer, fakeChild).
• Table-driven tests: Refactored golang_test.go into clean, table-driven test cases.
• Dependency cleanup: Dropped git.sr.ht/~nelsam/hel and github.com/nelsam/correct from go.mod and go.sum, as well as mock-generation build targets in Earthfile.

[github-actions]

🎉 Are we earthbuild yet?

Great progress! You've reduced "earthly" occurrences by 1 (0.02%)

📈 Overall Progress

Branch	Total Count
main	4752
This PR	4751
Difference	-1 (0.02%)

📁 Changes by file type:

File Type	Change
Go files (.go)	➖ No change
Documentation (.md)	➖ No change
Earthfiles	✅ -1

---

Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

Run locally to see detailed breakdown:

./.github/scripts/count-earthly.sh

Note that the goal is not to reach 0.
There is anticipated to be at least some occurences of earthly in the source code due to backwards compatibility with config files and language constructs.