Fix resolution of field type when it is the same as the field name

[andrewhickman]

Fixes #99

[codecov]

Codecov Report

Attention: Patch coverage is 80.95238% with 28 lines in your changes are missing coverage. Please review.

Project coverage is 76.65%. Comparing base (d554c53) to head (c25b777).

Files	Patch %	Lines
prost-reflect/src/descriptor/build/mod.rs	82.27%	14 Missing ⚠️
prost-reflect/src/descriptor/build/resolve.rs	83.33%	7 Missing ⚠️
prost-reflect/src/descriptor/build/options.rs	80.00%	3 Missing ⚠️
prost-reflect/src/dynamic/serde/ser/mod.rs	0.00%	2 Missing ⚠️
prost-reflect/src/dynamic/fields.rs	50.00%	1 Missing ⚠️
prost-reflect/src/dynamic/text_format/format.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
+ Coverage   76.30%   76.65%   +0.34%     
==========================================
  Files          31       31              
  Lines        5452     5439      -13     
==========================================
+ Hits         4160     4169       +9     
+ Misses       1292     1270      -22     

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