pyOCD fails without cmsis-pack-manager despite docs suggesting it's optional

The docs suggest that pyOCD can be used without cmsis-pack-manager (https://pyocd.io/docs/installing_on_non_x86.html), but I've observed several failures when it isn't installed.

The first error is due to an issue with type annotations

$ python
Python 3.9.2 (default, Mar 12 2021, 04:06:34) 
[GCC 10.2.1 20210110] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pyocd
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.9/dist-packages/pyocd/__init__.py", line 21, in <module>
    from . import gdbserver
  File "/usr/local/lib/python3.9/dist-packages/pyocd/gdbserver/__init__.py", line 17, in <module>
    from .gdbserver import GDBServer
  File "/usr/local/lib/python3.9/dist-packages/pyocd/gdbserver/gdbserver.py", line 37, in <module>
    from .syscall import GDBSyscallIOHandler
  File "/usr/local/lib/python3.9/dist-packages/pyocd/gdbserver/syscall.py", line 18, in <module>
    from ..debug.semihost import SemihostIOHandler
  File "/usr/local/lib/python3.9/dist-packages/pyocd/debug/semihost.py", line 26, in <module>
    from ..core import (exceptions, session)
  File "/usr/local/lib/python3.9/dist-packages/pyocd/core/session.py", line 30, in <module>
    from ..board.board import Board
  File "/usr/local/lib/python3.9/dist-packages/pyocd/board/board.py", line 23, in <module>
    from ..target.pack import pack_target
  File "/usr/local/lib/python3.9/dist-packages/pyocd/target/pack/pack_target.py", line 55, in <module>
    class ManagedPacksImpl:
  File "/usr/local/lib/python3.9/dist-packages/pyocd/target/pack/pack_target.py", line 64, in ManagedPacksImpl
    def get_installed_packs(cache: Optional[cmsis_pack_manager.Cache] = None) -> List["CmsisPackRef"]: # type:ignore
NameError: name 'cmsis_pack_manager' is not defined

This issue can be fixed by adding from __future__ import annotations to pyocd/target/pack/pack_target.py as suggested here: https://mypy.readthedocs.io/en/stable/runtime_troubles.html#future-annotations-import-pep-563

After fixing that error, the following issue is observed

$ python -m pyocd
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/kevin/code/pyOCD/pyocd/__init__.py", line 21, in <module>
    from . import gdbserver
  File "/Users/kevin/code/pyOCD/pyocd/gdbserver/__init__.py", line 17, in <module>
    from .gdbserver import GDBServer
  File "/Users/kevin/code/pyOCD/pyocd/gdbserver/gdbserver.py", line 41, in <module>
    from ..rtos import RTOS
  File "/Users/kevin/code/pyOCD/pyocd/rtos/__init__.py", line 29, in <module>
    load_plugin_classes_of_type('pyocd.rtos', RTOS, ThreadProvider)
  File "/Users/kevin/code/pyOCD/pyocd/core/plugin.py", line 97, in load_plugin_classes_of_type
    plugin = entry_point.load()()
  File "/Users/kevin/code/pyOCD/venv/lib/python3.9/site-packages/pkg_resources/__init__.py", line 2464, in load
    self.require(*args, **kwargs)
  File "/Users/kevin/code/pyOCD/venv/lib/python3.9/site-packages/pkg_resources/__init__.py", line 2487, in require
    items = working_set.resolve(reqs, env, installer, extras=self.extras)
  File "/Users/kevin/code/pyOCD/venv/lib/python3.9/site-packages/pkg_resources/__init__.py", line 777, in resolve
    raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'cmsis-pack-manager<1.0,>=0.4.0' distribution was not found and is required by the application

The docs suggest it should be runnable through Python

Once pyOCD is successfully installed, you will need to run it as a module using the python executable as python -mpyocd rather than running the pyocd executable directly. This is because the pyocd executable, which is auto-generated by the install process, verifies dependencies and will error out due to the missing cmsis-pack-manager. But when run through python, dependencies are not checked and pyOCD can handle the lack of cmsis-pack-manager gracefully.

Start with pyocd/target/pack/pack_target.py and trace the optional cmsis-pack-manager reference shown in the import traceback. Then inspect pyocd/rtos/init.py and pyocd/core/plugin.py to understand why plugin loading still requires the missing distribution. Run python -m pyocd without cmsis-pack-manager and verify the documented usage works without these failures.