refactor - 2: move internal gitutil and builtins packages to pkg/lib/

[aravindtga]

Description

This PR continues the refactoring effort by moving two additional internal packages (gitutil and builtins) from internal/ to pkg/lib/ to expose them as part of the publicly accessible Library. Additionally, it improves the package-context.yaml generation logic to be more precise. This is the second phase of the broader refactoring effort to make key utilities available to external consumers.

Changes

Move:

• internal/gitutil/-> pkg/lib/gitutil/
• internal/builtins/ -> pkg/lib/builtins/

Update:

• import statements across multiple command and internal files to reference the new public library locations
• Add validation to ensure only one package-context.yaml per kpt package

Future Work

This continues the systematic refactoring to move internal packages to the public library. Additional internal packages will be evaluated and moved in subsequent phases to create a comprehensive and accessible library interface for kpt consumers.

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