Make sure correct dependencies are selected if implementation_name environment marker is used

[aivarannamaa]

Currently pip does not have means for overriding the value of implementation_name when filtering dependencies. In particular, --implementation option does not work for this. See pypa/pip#9981 for discussion.

I suppose this issue can't progress until pip starts supporting overriding markers. I could try patching sys.implementation.name, but even if it works with current version of pip, this hack is not guaranteed to work in the future.