dag: do not include DVC-tracked dependencies as stages

[shcheklein]

DVC version:

DVC version: 1.0.0a10+4042d5
Python version: 3.7.7
Platform: Darwin-19.4.0-x86_64-i386-64bit
Binary: False
Package: None
Supported remotes: azure, gdrive, gs, hdfs, http, https, s3, ssh, oss
Cache: reflink - supported, hardlink - supported, symlink - supported
Repo: dvc, git

Reproduce:

clone the https://github.com/shcheklein/example-get-started, run dvc dag

Issue:

We print something like this:

   +-------------------+
    | data/data.xml.dvc  |
    +------------------+
              *
              *
              *
         +---------+
         | prepare   |
         +---------+
              *
              *
              *
        +-----------+
        | featurize    |
        +-----------+
         **        **
       **            *
      *               **
+-------+             *
|   train   |             **
+-------+            *
         **        **
           **    **
             *  *
        +----------+
        | evaluate   |
        +----------+

It's not clear why we consider data dependency a stage. Why do we consider only DVC-tracked or imported deps as stages, not any other deps?

Suggestion is to avoid showing any dependencies (at least in the default view) and focus on stages only.

[efiop]

Closing as stale.