Unpacking should intersect constraint when there is already one

[stof]

See https://github.com/ro0NL/sf-project/commit/b204330a1f053f530b930bf8710dd36cdca5e85e

When having a root constraint for a package and unpacking with another constraint on the same package, we cannot just use the constraint for the pack. We need to take the intersection of both constraints.

[stof]

And the same is true for conflict rules (after fixing #371), but using the union of ranges (which is easier, as you can simply join both constraints with ||, even if it may not be the simpler constraint)

[ro0NL]

Just got the same

-        "symfony/validator": "^4.1",
+        "symfony/validator": "^3.0 || ^4.0",

To make it more easy, we could skip existing constraints and show a log or so.

[nicolas-grekas]

Skipping looks like the proper simple approach to me. PR welcome.

[stof]

Well, for requirements, skipping might be an option (even though it might still create issues by broadening the constraint). But for conflict rules, it would end up with missing conflicting versions.

[nicolas-grekas]

Closing in favor of #645

[nicolas-grekas]

Reopening since the logic in place didn't work as expected and was reverted in #695
See also #694

Help wanted.

[nicolas-grekas]

Closing as there is little incentive to work on this.
PR welcome when this incentive is high for you.