The conditional for DEVELOPER_MODE is too widely-scoped, as source distributions (sdists) are released to end-users, and the presence of a MANIFEST.in shouldn't mean DEVELOPER_MODE.

A slightly better form might be to check for the presence of a git checkout, though this is also problematic, as users can depend on packages via their git URL's.

The check should probably be an explicit opt-in via the presence of an environment variable DEVELOPER_MODE=yes or similar.