refactor: split go mod into 2 pipes

[caarlos0]

split the gomod pipe into 2:

• one just loads the go module information (aka set ctx.ModulePath)
• the other, if enabled, proxies the modules

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[codecov]

Codecov Report

Merging #2502 (272e7e8) into master (28ef87b) will decrease coverage by 0.01%.
The diff coverage is 73.40%.

@@            Coverage Diff             @@
##           master    #2502      +/-   ##
==========================================
- Coverage   83.40%   83.39%   -0.02%     
==========================================
  Files          92       93       +1     
  Lines        6467     6462       -5     
==========================================
- Hits         5394     5389       -5     
  Misses        916      916              
  Partials      157      157              

Impacted Files	Coverage Δ
internal/pipe/gomod/gomod.go	100.00% <ø> (+21.55%)	⬆️
internal/pipe/gomod/gomod proxy.go	73.40% <73.40%> (ø)

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