Only return external output resolver if the input resolver also goes to the external

[DavyLandman]

This would lead to subtle bugs with write functions failing in ways that aren't expected.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 46%. Comparing base (6cff72e) to head (65da501).

Files with missing lines	Patch %	Lines
src/org/rascalmpl/uri/URIResolverRegistry.java	50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              main   #2804   +/-   ##
=======================================
  Coverage       46%     46%           
- Complexity    6722    6729    +7     
=======================================
  Files          839     839           
  Lines        66766   66767    +1     
  Branches      9983    9983           
=======================================
+ Hits         30782   30794   +12     
+ Misses       33601   33595    -6     
+ Partials      2383    2378    -5     

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