Fix writer order preference

[tlambert03]

fixes #129, ensuring that – all else being equal – writers declared earlier in a plugin's manifest have priority over later ones

[codecov]

Codecov Report

Merging #172 (7d89e4d) into main (a483aea) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #172   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines         1730      1728    -2     
=========================================
- Hits          1730      1728    -2     

Impacted Files	Coverage Δ
npe2/_plugin_manager.py	100.00% <100.00%> (ø)

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[nclack]

thanks! Looks like there's some dead code that should be removed.