Remove restriction to local externs ECFLOW-2033

[marcosbento]

Description

These changes remove restriction to local externs, thus allowing the declaration of externs that declare nodes/attributes to a suite that is locally available.

For example, the following will be allowed (where it was previously not accepted):

# notice that the /user suite is defined "partially". This means that only the branch
# /user/experiment2 is available locally, while the user obviously expects to also have
# a /user/experiment1 available on the server.

extern /user/experiment1/path/to/specific/node
suite user
  family experiment2
    family node
      trigger /user/experiment1/path/to/specific/node == complete
    endfamily
  endfamily
endsuite

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌦️ >> Documentation << 🌦️
https://sites.ecmwf.int/docs/dev-section/ecflow/pull-requests/PR-264

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.25%. Comparing base (d2b2d45) to head (05617ff).

Files with missing lines	Patch %	Lines
libs/test/foolproof/TestFoolproof.NodeExtern.cpp	95.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #264      +/-   ##
===========================================
- Coverage    49.29%   49.25%   -0.04%     
===========================================
  Files         1230     1231       +1     
  Lines        96924    96964      +40     
  Branches     14858    14857       -1     
===========================================
- Hits         47778    47762      -16     
- Misses       49146    49202      +56     

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