distinguish PEP440 filtering from pre-release filtering

[qwcode]

for reference see #1492

the prerelease exclusion filter added in v1.4 also acts as a PEP440 filter (or at least a filter against anything that can't be normalized to PEP440 using distlib's "suggest" feature)

we need to backfill the 1.4 changelog (and update our docs) to make this clear.

going forward, we should make the PEP440 filter and the prerelease filter independent and distinguish the warnings for clarity. to do so, we'll add a new --allow-nonstandard-version (or --allow-legacy-version), to turn off the PEP440 filter.

This will mean a change in behavior for --pre. By itself, it won't turn on something like 1.3-fork1 anymore. that would require --allow-nonstandard-version.

[dstufft]

+1

[techtonik]

I'd also vote to provide opt-out ability to package authors from this intelligence that really gets in a way.

[techtonik]

The package should have the ability to choose filtering method itself.

[dstufft]

This is made invalid by bff1145.

[qwcode]

yay!