Warn users when traversing special directories, recommend to ignore

[skshetry]

Currently, we ignore .dvc, .git and .hg directories by default. Still, we might be traversing through other special directories such as: node_modules, .venv/venv/env, __pycache__, dist/eggs/wheels, htmlcov, docs, .mypy_cache, .pytest_cache, etc.

We could throw warning if the users have not ignored these kind of directories and recommend them to add to .dvcignore. We could start with a small set of such directories.

Context:

Exactly. There can be tons of special/hidden dirs you don't want DVC to traverse (.idea/ .project/
.env/ .ds_store/ venv/), and you can just add them to .gitignore or .dvcignore. Why are we secretly
integrating with Mercurial if we don't actually support it? What if someone wants to version a .hg
dir? (edge case I know)

It just seems very arbitrary to me, but OK maybe I'm missing something...

Originally posted by @jorgeorpinel in #3257 (comment)

[shcheklein]

@skshetry please, consider adding a link to the discussion - I think it can be valuable.