repo: abstract erepo dvc/git handling into RepoTree

[pmrowla]

Issue for keeping track of what needs to be done for the tree abstraction

main tasks:

• Support fetch/stream on open() (repo: Support streaming and pulling files on RepoTree/DvcTree.open() #3810)
• Support saving from a tree directly to cache via walk()+open() (remote: support saving RepoTree objects directly to cache #3825)
• Rework erepo to use new tree functionality (external_repo: use RepoTree for fetch/import/get #3841)

other tasks:

• dvc list should use RepoTree (external_repo: use RepoTree for fetch/import/get #3841)

issues that should be fixed by this: #3611, #3087

[efiop]

Looks like we are done here. Feel free to reopen if i've missed anything.