fix: 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 95.00%. Comparing base (d3596c5) to head (763a49c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1315   +/-   ##
=======================================
  Coverage   95.00%   95.00%           
=======================================
  Files          42       42           
  Lines        2762     2764    +2     
=======================================
+ Hits         2624     2626    +2     
  Misses        138      138           

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[abbiemery]

Now I understand the problem I think this is a valid improvement for now. It is worth us figuring out what approach we want to take re finding and returning devices going forward.