Added an internal pkg for the builder

[lonnblad]

No description provided.

[codecov]

Codecov Report

Merging #327 into master will decrease coverage by 0.37%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master     #327      +/-   ##
==========================================
- Coverage   87.51%   87.14%   -0.38%     
==========================================
  Files          22       22              
  Lines        2227     2225       -2     
==========================================
- Hits         1949     1939      -10     
- Misses        185      192       +7     
- Partials       93       94       +1     

Impacted Files	Coverage Δ
internal/builder/ast.go	83.33% <ø> (ø)
test_context.go	61.70% <0.00%> (-2.75%)	⬇️
internal/builder/builder.go	62.67% <100.00%> (ø)
run.go	81.88% <100.00%> (+0.14%)	⬆️

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