That is a good point :)
Long story ahead:
To start off, you must specify all necessary python modules in `WHEELS` in Makefile, otherwise the `requirements.txt` will not contain that module, and the wheel is not added into the SPK. (second option is to specify a predefined `requirements.txt`, but result is the same)

The potential issue is where you have two SPKs specifying the same module. 
- The first use-case for wheel is replacing the current pybundles, in which the uninstall would be more like `rm -rf /path/to/venv`. There, we have no issues, as it's isolated from other SPKs.
- Second use-case is creating wheels for modules that we currently add as `python-module`dependency in Python, and add those wheels to specific SPKs. Same as above, make sure those are added into venvs.
For the cases where the module is much used, such as openssl (?), and more of a necessity: I suggest to keep them in Python as `python-module`.

Specific to the Mercurial SPK, it so happens that it's the only req. (It does use docutils to create the wheel, but that is not a requirement for installation)
In addition, I purposely installed mercurial into python system-site, for somewhat easier access, but otherwise, it would have been installed in a venv.