(PDK-1199) Honour .{pdk,git}ignore in check:symlinks rake task

[rodjek]

Introduces pathspec as a dependency (pure ruby and is already
a dependency of PDK and Puppet so it shouldn't be an issue). Also moves
the symlink checking logic out of the existing helper module and into
its own class for easier isolation between test cases.

[codecov-io]

Codecov Report

Merging #267 into master will increase coverage by 2.76%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #267      +/-   ##
==========================================
+ Coverage   41.61%   44.37%   +2.76%     
==========================================
  Files          10       11       +1     
  Lines         757      773      +16     
==========================================
+ Hits          315      343      +28     
+ Misses        442      430      -12

Impacted Files	Coverage Δ
lib/puppetlabs_spec_helper/tasks/fixtures.rb	35.04% <ø> (+0.96%)	⬆️
lib/puppetlabs_spec_helper/rake_tasks.rb	46.07% <100%> (+2.23%)	⬆️
lib/puppetlabs_spec_helper/tasks/check_symlinks.rb	100% <100%> (ø)

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[puppetcla]

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