adding build-scoping for builds on earlier go versions (<1.12)

[marcusljx]

Fixes #1061

This PR moves the helper function using strings.ReplaceAll to a new file which has build tags for go.1.12, limiting it to go1.12+ builds. Also added a file for go versions lesser than 1.12, using strings.Replace in place of strings.ReplaceAll.

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[marcusljx]

@googlebot I signed it!

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[johanbrandhorst]

Hi @marcusljx, this looks great. I think you need to run the generator for the bazel files. See CONTRIBUTING.md.

[achew22]

Or if you want to do it manually, this file is a list of the .go files used in the build

[codecov-io]

Codecov Report

Merging #1062 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #1062   +/-   ##
=======================================
  Coverage   53.84%   53.84%           
=======================================
  Files          41       42    +1     
  Lines        4147     4147           
=======================================
  Hits         2233     2233           
  Misses       1670     1670           
  Partials      244      244

Impacted Files	Coverage Δ
protoc-gen-swagger/genswagger/generator.go	10.15% <ø> (+0.15%)	⬆️
protoc-gen-swagger/genswagger/helpers.go	0% <0%> (ø)

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[marcusljx]

Thanks @johanbrandhorst and @achew22! I ended up going with the manual route, as I kept getting a make:generate command not found error with the first command from the CONTRIBUTING file (docker run ....).

[achew22]

@marcusljx, thanks so much for fixing up the BUILD files! This looks good to me. I'm going to let @johanbrandhorst take one more look at it and merge it but looks good on my front.

[johanbrandhorst]

Thanks for your contribution!