dvc.objects: use stub ignore in dvc.objects/dvc.data

This commit:
* defines a `Ignore` protocol in `dvc.objects`, that expects a `find` method.
  This will be used in dvc-data and dvc-objects instead of depending on dvc.ignore.
  dvcignore is expected to respect this protocol, which it does.
* renames `dvcignore` kwarg to `ignore` in `dvc-data` and `dvc-objects`.
* teaches dvc.cli that `IgnoreInCollectedError` is a known exception
   and is logged correctly and adds a test for the behaviour.

I am not happy that we need to hardcode `.dvcignore` in dvc-data
but I don't see any easy way, since we always try to avoid
staging if the tree contains .dvcignore file, even if .dvcignore
is not passed.

tests/unit/cli/test_main.py

@@ -4,6 +4,7 @@ import pytest
 from funcy import raiser
 
 from dvc.cli import main
+from dvc.data.stage import IgnoreInCollectedDirError
 from dvc.objects.cache import DiskError
 from dvc.objects.fs.base import FileSystem, RemoteMissingDepsError
 
@@ -68,3 +69,18 @@ def test_remote_missing_deps_are_correctly_reported(
             "Thank you!"
         )
     assert expected in caplog.text
+
+
+def test_ignore_in_collected_dir_error_is_logged(tmp_dir, caplog, mocker):
+    error = IgnoreInCollectedDirError(".dvcignore", "dir")
+    mocker.patch(
+        "dvc.cli.parse_args",
+        return_value=Namespace(
+            func=raiser(error),
+            quiet=False,
+            verbose=True,
+        ),
+    )
+    assert main() == 255
+    expected = ".dvcignore file should not be in collected dir path: 'dir'"
+    assert expected in caplog.text