chore: extract Provider piece logic to separate file

[rvagg]

Based on feedback in #749, which applied to #747 which was already merged.

also includes:

• fix: handle error AND zero-length cases from piece lookup

[codecov-commenter]

Codecov Report

Merging #750 (e051f00) into master (787c67b) will increase coverage by 0.38%.
The diff coverage is 71.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #750      +/-   ##
==========================================
+ Coverage   57.59%   57.97%   +0.38%     
==========================================
  Files          65       66       +1     
  Lines        5474     5540      +66     
==========================================
+ Hits         3152     3211      +59     
- Misses       1973     1979       +6     
- Partials      349      350       +1     

Impacted Files	Coverage Δ
storagemarket/impl/provider_environments.go	10.38% <0.00%> (+3.11%)	⬆️
storagemarket/impl/providerstates/provider_fsm.go	74.60% <ø> (ø)
storagemarket/impl/provider.go	26.50% <7.15%> (-0.13%)	⬇️
retrievalmarket/impl/provider_environments.go	66.67% <46.16%> (-2.38%)	⬇️
retrievalmarket/impl/provider.go	68.33% <66.67%> (-1.20%)	⬇️
retrievalmarket/impl/provider_pieces.go	83.19% <83.19%> (ø)