refactor: convert "link tuple" to FoundVersion class

[bukzor]

This changeset builds on top of buck-cachedproperty.
See the diff here: bukzor/pip@buck-cachedproperty...buck-found-versions

This makes things significantly simpler and more readable, and paves the way for more significant simplifications.

[bukzor]

I've rebased to buck-cachedproperty. This should make review/merge easier.

[bukzor]

This is good to go imo.

[bukzor]

I still believe this is an improvement to readability, understandability, refactorability.

[msabramo]

This does look a bit more readable and easier to factor from a quick glance. Doesn't merge clean now though.

[dstufft]

Accidentally closed this, reopening. Sorry!

[BrownTruck]

Hello!

As part of an effort to ease the contribution process and adopt a more standard workflow pip has switched to doing development on the master branch. However, this Pull Request was made against the develop branch so it will need to be resubmitted against master. Unfortunately, this pull request does not cleanly merge against the current master branch.

If you do nothing, this Pull Request will be automatically closed by @BrownTruck since it cannot be merged.

If this pull request is still valid, please rebase it against master (or merge master into it) and resubmit it against the master branch, closing and referencing the original Pull Request.

If you choose to rebase/merge and resubmit this Pull Request, here is an example message that you can copy and paste:

This changeset builds on top of buck-cachedproperty.
See the diff here: bukzor@buck-cachedproperty...buck-found-versions

This makes things significantly simpler and more readable, and paves the way for more significant simplifications.

---

*This was migrated from pypa/pip#2054 to reparent it to the ``master`` branch. Please see original pull request for any previous discussion.*

[BrownTruck]

This Pull Request was closed because it cannot be automatically reparented to the master branch and it appears to have bit rotted.

Please feel free to re-open it or re-submit it if it is still valid and you have rebased it onto master or merged master into it.