[WIP] Add feature to configure build plan download URL.

[igrep]

Fixes #1794

Left TODOs

• Test (Really necessary?)
• Rename/Restructure the new configuration item if you can find better ones.
  • It may not have to be split into lts and nightly.
• Documentation (I want to write after you approve).
• ChangeLog.

Feel free to advice me on the source code structure, the new configuration item, and any others!

[mgsloan]

Since we may want to configure quite a few different URLs, why not just call this StackURLs? The fields can specify that they are prefixes. This would correspond to the configuration file option

urls:
    lts-prefix: ...
    nightly-prefix: ...
    ...

Also, I don't see actual usage of this in the diff, just types and commandline parsing.

[igrep]

Since we may want to configure quite a few different URLs, why not just call this StackURLs?

You mean I should unify the configurations like download-prefix in package-indices and latest-snapshot-url into StackURL?
Sounds good 🌟 , but I have to consider stack.yaml's compatibility doing that.
Can I make a separate pull request to make a new configuration item to unify them?
I guess the change would be very big.

Also, I don't see actual usage of this in the diff, just types and commandline parsing.

OMG! 😱 Seems I've deleted the change by mistake! Sorry, fix soon.

[igrep]

Fixed and rebased!

[mgsloan]

We can just deprecate the latest-snapshot-url field and add it to this urls list. I think the indices urls can be left where they are

[mgsloan]

Oops sorry for letting this languish. Hadn't seen the new commit. LGTM, thanks a bunch!

[TerrorJack]

Thank you for this awesome work!

[igrep]

Oh my... 😲 I was actually in progress.
I was planning to push 5874b9e ...
But, Okay! Thanks!

[mgsloan]

Oh, sorry, I was in PR cleanup mode