u3d/unity_versions: list from additional U3D_ADDITIONAL_PAGES

[niezbop]

Pull Request Checklist

• My pull request has been rebased on master
• I ran bundle exec rspec to make sure that my PR didn't break any test
• I ran bundle exec rubocop to make sure that my PR is inline with our code style
• I have read the code of conduct

Pull Request Description

This adds the ability to list unity versions from sources other than the one preconfigurated in u3d, through an environment variable U3D_ADDITIONAL_PAGES. This allows for greater flexibility and compensates recent changes on the Unity website that prevent us from listing beta versions.

Sample output:

$ U3D_ADDITIONAL_PAGES=https://unity3d.com/unity/beta/2018.3 u3d available -f --no-central
Cache is out of date. Updating cache for Windows
Loading Unity lts releases
Found 17 lts releases.
Loading Unity stable releases
Found 105 stable releases.
Loading Unity patch releases
Found 191 patch releases.
Loading Unity beta releases
Found 1 beta releases.
Loading Unity custom releases
Found 1 custom releases.
[...]
Version 2018.2.16f1: https://download.unity3d.com/download_unity/39a4ac3d51f6/
Version 2018.2.17f1: https://download.unity3d.com/download_unity/88933597c842/
Version 2018.2.18f1: https://netstorage.unity3d.com/unity/4550892b6062/
Version 2018.3.0b12: http://beta.unity3d.com/download/77f6238a7ced/

Note the last build, the 2018.3.0b12 which was not accessible before that.

[niezbop]

This could address the issues brought in #330 and related

[niezbop]

This is kind of replaced by #335 as a fix for #330 but this still could be a useful feature

[KonH]

Hello, @niezbop! As I see, this PR can resolve this issue - #360. Unfortunately, it isn't merged yet and it looks like the project is abandoned or something like that. I will try to build it from scratch based on your branch (but don't have experience with it), but it will be helpful if this PR will be merged in the future.

[KonH]

I've tried your branch, but it shows less available versions (I add U3D_ADDITIONAL_PAGES=https://unity3d.com/unity/beta/2018.3 to get previously available, but can't get wanted versions).
Then I've tried to cherry-pick your changes over this repo master branch, but didn't sure how exactly I need to solve merge conflicts.
Can you please give me some advices? My goal is get 2019.1+ versions on Linux, which isn't available now.

[niezbop]

Hey @KonH ! The project is still alive! We are still using u3d on a daily basis and have no plan to drop support for it 😄 Unfortunately we have had tight deadlines to hold since and no time to spend on this project.
I'll take a swing at this soon, and hopefully solve a bit your issue.

[niezbop]

So the pull request has no longer any conflicts. It's still not complete in my mind since it does not support Linux yet. I'll open a quick fix for #360 using another strategy, but this could still be finished. Or at least should be closed. WDYT @lacostej ?