feat: allow overriding namespace when calling Scan and Resolve

[ggicci]

also...

1. removed the ReplaceDirectiveExecutor API
2. removed some error constants

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (ff1212b) 100.00% compared to head (f6b3757) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #5   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          381       382    +1     
=========================================
+ Hits           381       382    +1     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
errors.go	100.00% <100.00%> (ø)
namespace.go	100.00% <100.00%> (ø)
resolver.go	100.00% <100.00%> (ø)

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