compose-go's loader now silently renames both deprecated
x-initialSync and x-develop extensions to their official
initial_sync and develop attributes before compose ever sees them
as extensions, so the legacy lookups and deprecation warnings for
both can no longer be reached. That's why the e2e test asserting on
the x-initialSync warning broke under the bump.
Simplify both lookups to rely on the promoted fields directly, and
rewrite the e2e test to check the legacy attribute still triggers a
real initial sync instead of asserting on an unreachable message.
Signed-off-by: Guillaume Lours <[email protected]>