DiscreETLy (tables) "uses" field can't reference tables of other DAGs

[MikolajBalcerek]

Describe the bug
Tables extension doesn't properly support tables relying ("uses") on other tables from different DAG.
This causes issues on "Tables managed by ..." view, where no graph will be shown for the 'dag2' (see Steps to reproduce).

To Reproduce
Steps to reproduce the behavior:

1. In /config/tables.yaml declare two tables, belonging to two different DAGs, where one is using another e.g.:

- name: table1
  db: db
  dag_id: dag1
  task_id: create_table1

- name: rollup_of_table1
  db: db
  uses: db.table1
  dag_id: dag2
  task_id: rollup_of_table
```1

2. Run DiscreETLy, go to /etl, select the graph view for dag2
3. No graph is shown

**Expected behavior**
The graph should be shown.

[MikolajBalcerek]

See #23 for an introduction of a workaround - allowing for declaration and specified above, but losing progress and dependency tracking on Tables Managed by..

More extensive rewrite would have to happen to properly support shared tables among different DAGs.