`import`: get from source project cache if `url/path` are in the local system

[jorgeorpinel]

UPDATE: Jump to #2599 (comment) (notice also #2599 (comment))

DVC version: 0.62.1
Python version: 3.7.3
Platform: Darwin-18.7.0-x86_64-i386-64bit
Binary: False
Filesystem type (workspace): ('apfs', '/dev/disk1s1')

I create a DVC project in /path/to/data-reg and dvc add a data/ directory in it.

Then I create another DVC project in /path/to/project in the same file system and try to import data/ but get:

$ dvc import /path/to/data-reg data
Importing 'data (../data-reg-test)' -> 'data'
Missing cache for directory '../../../../private/var/folders/_c/3mt_xn_d4xl2ddsx2m98h_r40000gn/T/tmpkq6tyek3dvc-repo/data'.
Cache for files inside will be lost. Would you like to continue? Use '-f' to force. [y/n] y
ERROR: failed to import 'data' from '/path/to/data-reg'. - config file error: no remote specified. Setup default remote with
    dvc config core.remote <name>
or use:
    dvc pull -r <name>


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A few problems:

• The first part of the message is completely mysterious to me. I don't understand what cache dir is missing and obviously that path isn't friendly.
• I also don't know what it's asking me to confirm so I just say y and move on 😋
• The "no remote specified" is also kind of cryptic (doesn't specify which project is missing a remote) but OK, I get it: I didn't push data/ to a remote in the first project, so the new project can't download it. However (main point of this issue) it could simply copy it from the source project's cache, since they're on the same file system. This would be a nice-to-have feature I think. In fact it should probably try this before the default remote, for performance.
• There's 2 new lines after dvc pull -r <name> in the error output.

UPDATE: Moved all the UI-related stuff to #2602 (per Ivan's #2599 (comment) below).

[shcheklein]

@jorgeorpinel I would probably split this into two separate issues - UI and optimization. It would be easier to track and discuss them.

[efiop]

Let's start with local, as it makes the most sense and is a natural thing to do. We need to at least show a warning that we are automatically using project's cache as a remote or maybe we need an explicit flag that would tell import/get that this is okay to do. On one hand, warning is a low-friction approach, but might be dangerous if user doesn't realize that he is not using a remote and then some colleague on another machine tries to dvc pull and there is no remote to pull imported data from. But, in that case the url of the git is a local url, that is not accessible on another machine, so user will run into that anyways. So maybe warning is enough. On another hand, an explicit flag would be a pain on dvc pull, as we will have to explicitly allow pulling from cache. Seems like the first approach is better.

[jorgeorpinel]

Let's start with local

You mean use local repo and/or cache as default place to get/import from? I would agree with this.

show a warning that we are automatically using project's cache... might be dangerous if user doesn't realize that he is not using a remote...

Maybe either way output where the import is coming from as a regular INFO message. But honestly, the possibility that people won't pay attention to the output is there anyway. Also, in some other recent discussion about adding output warnings someone brought up that they can break shell scripts and cause other problems unless hidden under the -v output.