remote: separate remote/cache/tree into distinct concepts

[pmrowla]

Currently, Remote classes can contain methods that fall into three categories:

• remote operations - indexing/gc/etc
• cache operations - save/checkout/link/etc
• tree operations - exists/isfile/isdir/upload/download/etc

Having all of these methods grouped into a single Remote class causes issues when we are dealing with paths that can come from different namespaces (trees). In particular, LocalRemote was originally written with the assumption that all paths for local remote or local cache operations are always filesystem paths, which is no longer the case given that we now need to support GitTree objects (via brancher and erepos).

Ex. LocalRemote.exists(path) is used in different places as a replacement for os.path.exists() to test if path exists in a local remote, if path exists in the local cache directory, and if path exists in a DVC repo, even though these are distinct operations.

Tasks:

• separate remote/cache/tree methods
  • tree methods -> RemoteTree (remote: separate tree functions from main remote classes #3931, remote: finish separating RemoteTree methods #3946)
  • cache methods (remote: separate cloud remote and cloud cache classes #4019, remote: continue remote/cache/tree refactoring #4041)
• cache methods which take path_info parameters which can belong to a namespace other than remote.tree should take explicit tree parameters, so it is always clear what tree path_info applies to (remote: separate cloud remote and cloud cache classes #4019)
• local cache should have its own tree instance that is separate from repo.tree (since local cache always operates on a filesystem working tree, but repo.tree does not) (remote: use separate working tree instance for local cache #3991)
• state should belong to local cache and not repo, and should only be used for working tree paths (remote: use separate working tree instance for local cache #3991)

[efiop]

remote operations - upload/download/etc

Maybe upload/download are actually related to the tree, but push/pull(aka fetch)/status are definitely remote concepts that will use upload/download in respective trees.

[pmrowla]

Some minor cleanup left w/improving remote function naming, but after #4041 this refactor is pretty much done for now.

#4050 has been filed as a potential follow up issue.