Fix Viewing Dag Code for Stateless Webserver

[kaxil]

This bug was found by @KostyaEsmukov

Details in #8151 (comment)

Basically when DagCode was instantiated it read the source code from Dag File, hence when the Webserver does not have access to the Dag File, it used to fail with FileNotFoundError:

>>> DagCode(dag_orm.fileloc)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 4, in __init__
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/state.py", line 433, in _initialize_instance
    manager.dispatch.init_failure(self, args, kwargs)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 69, in __exit__
    exc_value, with_traceback=exc_tb,
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 178, in raise_
    raise exception
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/state.py", line 430, in _initialize_instance
    return manager.original_init(*mixed[1:], **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/models/dagcode.py", line 59, in __init__
    self.source_code = DagCode._read_code(self.fileloc)
  File "/usr/local/lib/python3.7/site-packages/airflow/models/dagcode.py", line 63, in _read_code
    with open_maybe_zipped(fileloc, 'r') as source:
  File "/usr/local/lib/python3.7/site-packages/airflow/utils/file.py", line 102, in open_maybe_zipped
    return io.open(fileloc, mode=mode)
FileNotFoundError: [Errno 2] No such file or directory: '<...my_dag...>'

This PR fixes this and adds additional tests to validate this behavior

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[kaxil]

Note: This PR is for v1-10-test branch to fix 1.10.10rc3 . I will port this to Master branch once we release 1.10.10

[kaxil]

cc @anitakar

[mik-laj]

I don't like this method name. It describes what he does. It does not contain any verb. However, this is not related to this PR. It should be get_code_by_fileloc or something similar.

[kaxil]

I have a separate PR to improve the code in this file, that I have targetted against Master but yeah that is not the priority for now.