Remove dot imports

[adrg]

Removes dot imports from the model package. (I did not find any others using them)
Resolves #309

[codecov]

Codecov Report

Merging #341 into v3 will increase coverage by 0.02%.
The diff coverage is 53.33%.

@@            Coverage Diff             @@
##               v3     #341      +/-   ##
==========================================
+ Coverage   58.22%   58.25%   +0.02%     
==========================================
  Files         145      145              
  Lines       25231    25235       +4     
==========================================
+ Hits        14691    14700       +9     
  Misses      10170    10170              
+ Partials      370      365       -5

Impacted Files	Coverage Δ
pdf/model/pattern.go	0% <0%> (ø)	⬆️
pdf/model/shading.go	0% <0%> (ø)	⬆️
pdf/model/image.go	58.5% <100%> (-0.69%)	⬇️
pdf/model/structures.go	80% <100%> (+1.42%)	⬆️
pdf/model/resources.go	57.2% <56.36%> (+0.43%)	⬆️
pdf/model/page.go	56.81% <57.3%> (ø)	⬆️
pdf/model/xobject.go	51.6% <58.69%> (+1.16%)	⬆️
pdf/model/functions.go	48.89% <65.9%> (-0.67%)	⬇️
pdf/model/outlines.go	67.36% <80%> (ø)	⬆️
pdf/model/reader.go	66.44% <86.84%> (ø)	⬆️
... and 33 more

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