fix: Configure Codecov for mono-repo structure

[nielsenko]

Description

Codecov is reporting incorrect coverage (28%) after the recent mono-repo restructuring, when the actual coverage is above 90%. This PR fixes the Codecov configuration to properly handle the multi-package mono-repo structure.

• Added ignore paths to exclude non-package directories (examples, benchmark, doc) from coverage calculations.
• Split the single coverage upload into three separate uploads, each with the corresponding package flag (relic_core, relic_io, relic). This ensures Codecov correctly associates each coverage report with its package.

Related Issues

• Fixes: #?

Pre-Launch Checklist

Please ensure that your PR meets the following requirements before submitting:

• This update focuses on a single feature or bug fix. (For multiple fixes, please submit separate PRs.)
• I have read and followed the Dart Style Guide and formatted the code using dart format.
• I have referenced at least one issue this PR fixes or is related to.
• I have updated/added relevant documentation (doc comments with ///), ensuring consistency with existing project documentation.
• I have added new tests to verify the changes.
• All existing and new tests pass successfully.
• I have documented any breaking changes below.

Breaking Changes

• No breaking changes.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 27.95%. Comparing base (196eef7) to head (e1ad156).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #318   +/-   ##
=======================================
  Coverage   27.95%   27.95%           
=======================================
  Files          87       87           
  Lines        3291     3291           
  Branches     1735     1735           
=======================================
  Hits          920      920           
  Misses       2371     2371           

Flag	Coverage Δ
relic_core	27.95% <ø> (?)

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