[nat] Log warning when no path to test is included

[scharissis]

When a test is referenced without including its full package path NAT must search for it. This dramatically lengthens the runtime.
When this happens we should log a warning, ideally with the path to be included, as a prompt for the user to add it. Ideally this will remind them to add it before they merge the new test(s) to main.

Example (bad):

tests:
          - name: TestWallet

Example (good):

tests:
          - name: TestWallet
            package: github.com/ethereum-optimism/optimism/devnet-sdk/system