Validate relpath util

[mpacer]

This creates the utility that will be shared between the fs cloning handler & the fs cloning api handler:

• checks the behaviour of the basedir
• ensures relpath exists
• ensures relpath does escape out of the basedir

[codecov[bot]]

Codecov Report

Merging #157 into master will increase coverage by 0.51%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #157      +/-   ##
=========================================
+ Coverage   79.39%   79.9%   +0.51%     
=========================================
  Files          10      10              
  Lines         427     438      +11     
=========================================
+ Hits          339     350      +11     
  Misses         88      88

[codecov[bot]]

Codecov Report

Merging #157 into master will increase coverage by 0.65%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
+ Coverage   79.43%   80.09%   +0.65%     
==========================================
  Files          10       10              
  Lines         428      442      +14     
==========================================
+ Hits          340      354      +14     
  Misses         88       88

[mpacer]

@chyzzqo2 I think this addresses all your comments now.