Missing main function error should be moved into Nargo

[phated]

Problem

Currently, the compile_main function creates a diagnostic error if it a main function is missing; however, that error should only occur if the PackageType (as introduced by #2134) is Binary but not if it is Library or Contract.

There is a bit of tangled APIs that result in the current implementation that need to be refactored to make this possible.

Happy Case

The error should only happen if the PackageType is Binary, otherwise different errors would be given (such as "Missing contracts").

Alternatives Considered

No response

Additional Context

This will likely be included in the solution to #2075

Would you like to submit a PR for this Issue?

Yes

Support Needs

No response