run: introduce --always-changed

This is an extension of our callback stages (no deps, so always
considered as changed) for stages with dependencies, so that these
stages could be used properly in the middle of the DAG. See attached
issue for more info.

Related to #2378

tests/unit/command/test_run.py

@@ -32,6 +32,7 @@ def test_run(mocker, dvc_repo):
             "outs-persist",
             "--outs-persist-no-cache",
             "outs-persist-no-cache",
+            "--always-changed",
             "command",
         ]
     )
@@ -58,6 +59,7 @@ def test_run(mocker, dvc_repo):
         ignore_build_cache=True,
         remove_outs=True,
         no_commit=True,
+        always_changed=True,
         cmd="command",
     )
 
@@ -83,6 +85,7 @@ def test_run_args_from_cli(mocker, dvc_repo):
         ignore_build_cache=False,
         remove_outs=False,
         no_commit=False,
+        always_changed=False,
         cmd="echo foo",
     )
 
@@ -108,5 +111,6 @@ def test_run_args_with_spaces(mocker, dvc_repo):
         ignore_build_cache=False,
         remove_outs=False,
         no_commit=False,
+        always_changed=False,
         cmd='echo "foo bar"',
     )