Prevent loss of deep dependencies of local requirements

[AndydeCleyre]

Fixes #1505

Fixes #1054

I've just:

• restored the minimum use of copy_ireq_dependencies, which we removed as part of Ensure pip-tools unions dependencies of multiple declarations of a package with different extras #1486 for some important fixes
• integrated @microcat49's attribute-copying fix for lost info during combine_install_requirements from Fixing issue with losing data about local files #1385

@richafrank Please have a look, as I don't know what any negative consequences or better alternatives might be.

That said, all existing tests continue to pass, and the behavior regarding those bugs is looking good.

TODO:

• sanity check
• add tests

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[richafrank]

General idea looks right to me @AndydeCleyre ! Looking at the differences between yours and mine:

1. This is missing handling for the combined_ireq being a constraint.
2. This didn't restore the test for FakeRepository.
3. I added a new test for Second level dependencies missing on for local file packages on >= 6.3.1 #1505.

So basically the same!

As I mentioned in #1505 (comment), I think there may be another broken case that no one's complained about yet, so we might need to keep iterating on this. The various pieces of state here are very challenging...

[AndydeCleyre]

Closing in favor of #1507, thanks!