mxdev does not work with mxmake + sources in 5.0.0

[jensens]

while all is fine if used directly, it stopped working with mxmake producing

error: Couldn't parse requirement in `requirements-mxdev.txt` at position 246
  Caused by: Expected path (`./sources/edutap.wallet_apple`) to end in a supported file extension: `.whl`, `.tar.gz`, `.zip`, `.tar.bz2`, `.tar.lz`, `.tar.lzma`, `.tar.xz`, `.tar.zst`, `.tar`, `.tbz`, `.tgz`, `.tlz`, or `.txz`
./sources/edutap.wallet_apple
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

First analysis is the offline mode bug fix. now that we are really offline in this mode, the sources are not fetched in mxmakes 2 stage installation cycle. Thus the sources directory is empty. We need to mitigate this by not writing the sources -e sources/packagename in the requirements-mxdev.txt if they we not fetched, but place a comment there. Logging a warning for transparency too, stating about the offline mode.