RepoTree: add support for subrepo traversal

[skshetry]

Right now, no actual "functionality" is surfaced as
subrepos is always set to False by default for now.

Subrepos are collected dynamically and trie is created to track those repos inside RepoTree, and this does not incur any cost
as .dvcignore is shared among subrepos and RepoTree just utilizes them.

Possible alternative design

1. Create "MotherOfTree" that will be prefix-based and can switch between those just based on prefix. So, some separate tree class that works on top of RepoTree or GitTree for submodules.
   cons: Hard to traverse, especially heirarchial traversal required for dvc repos, etc.

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[skshetry]

@efiop, I cannot split this into smaller PR, though more than half of it is tests.

[pmrowla]

Design-wise I think this looks a lot better than the first PR, but the some of the function naming was a bit confusing at first.

It wasn't entirely clear to me that _get_repo() and _get_tree_pairs() were returning Repo objs and tree pairs for subrepos (when the specified paths are inside a subrepo) until I read through everything a few times. Even one-liner comments for those methods explaining things would probably work.

[efiop]

I agree with Peter, looks much better than the previous one. Though it is clear that some concepts are still under development, so let's treat this PR as an evolutionary and cleanup more on top.

[skshetry]

Tests are failing due to #4229, as now we create a ".dvcignore" on every dvc init. So these 3 tests should add ".dvcignore" in it's expectations.