Use find_device from device endpoint

[tpoliaw]

Using the find_device method instead of looking devices up in the map of
devices directly means child devices can be found, eg 'stage.x' could be
a valid device but isn't in the devices map in its own right.

There is still a bit of weirdness as querying /devices/stage.x will return a
DeviceModel with a name of stage-x but querying for stage-x will
fail with a 404.

Relates to #903 and #1137.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.75%. Comparing base (21daed5) to head (7cd7dda).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1315   +/-   ##
=======================================
  Coverage   94.74%   94.75%           
=======================================
  Files          42       42           
  Lines        2741     2743    +2     
=======================================
+ Hits         2597     2599    +2     
  Misses        144      144           

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