Normalize specifiers by sorting (#6333)
We currently normalize package and extra names and drop the whitespace
from version specifiers, but we were not normalizing the order of the
specifiers. By sorting them we match the behavior of `packaging` and
become independent of build backends reordering specifiers (#6332).
Surprisingly, the snapshot diff isn't large - most people were already
writing sorted specifiers. Still, this will lead to observable
differences in lockfiles between releases in cases where there are
entries in `requires-dist` that were not previously sorted (while the
total number of `requires-dist` is already small compared to the overall
lockfile).
