Fix: Keep resolving references until there are none left

[localheinz]

This PR

• assers that nested references are resolved
• resolves references until there are none left

Follows #47.

[codecov]

Codecov Report

Merging #49 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #49   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity      104    104           
=======================================
  Files            13     13           
  Lines           292    294    +2     
=======================================
+ Hits            292    294    +2

Impacted Files	Coverage Δ	Complexity Δ
src/SchemaNormalizer.php	100% <100%> (ø)	38 <0> (ø)	⬇️

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