[Distutils] PEP 440 ad PEP 508 requirement formats

[Paul Moore]

Pip refers to PEP 440 when defining the format of a requirement (see
https://pip.pypa.io/en/stable/reference/pip_install/#requirement-specifiers).
But PEP 508 *also* defines a requirement - the title implies that it's
for dependency specification, but the content of the PEP says "the
language defined is a compact line based format which is already in
widespread use in pip requirements files".

So what's the relationship between PEP 440 and PEP 508? Which one is
the definitive definition of what is a "requirement"? The "packaging"
library implements specifiers which conform (according to the docs) to
PEP 440.

IMO, we're in danger of switching from having too few standards to
having too many...

Can someone clarify?
Thanks,
Paul