Prevent copying mount point data when setting install_data to a non existing directory

[AugustoMagalhaes]

Issue
Resolves #9071

Approach
Includes additional validation for source path in EverestConfig to avoid copying data from root folder ("/") when install_data template render an invalid string. I'm not sure wether to add this validation actually, I've included in check_path_exists function in validation_utils.py but in the first commit there's a glimpse of how it could be if the validation was in InstallDataConfig as a pydantic validation of the source property itself. Just let me know which one you guys think fits better (or a new one maybe) and I modify it later.

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.74%. Comparing base (90d11ec) to head (edf3ce8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9137      +/-   ##
==========================================
- Coverage   90.76%   90.74%   -0.03%     
==========================================
  Files         351      351              
  Lines       21895    21895              
==========================================
- Hits        19874    19868       -6     
- Misses       2021     2027       +6     

Flag	Coverage Δ
cli-tests	39.23% <ø> (-0.01%)	⬇️
gui-tests	71.77% <ø> (+0.02%)	⬆️
performance-tests	49.35% <ø> (-0.02%)	⬇️
unit-tests	79.63% <ø> (-0.02%)	⬇️

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