refactor(scip): separate out files in mapper that depends on proto to another directory

[zerunz]

Description

Move all functions under src/ulsp/mapper that touch proto code into a separate sub-package, src/ulsp/mapper/idl.
This way:
src/ulsp/mapper only contains logic for LSP, JSON-RPC, and general mapping.
src/ulsp/mapper/idl contains the parts that translate between internal models and protobuf messages.
When a controller imports mapper, no proto code is compiled in.
Only code that explicitly imports mapper/idl will depend on the generated .pb.go files.
So that it won't cause proto: duplicate proto type registered: error log when importing github.com/uber/scip-lsp/src/ulsp/controller/scip due to importing two versions of the same proto file that may conflict with each other

Also checked build graph:

$ bazel query "somepath(//src/ulsp/controller/scip:go_default_library, //idl/...)" 
INFO: Empty results

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test improvement

Component(s) Affected

• Language Server (ULSP)
• SCIP Generation (Python utilities)
• VS Code/Cursor Extension
• Java Aggregator
• Build System (Bazel)
• Documentation
• Tests

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes (bazel test //...)
• I have tested this manually with a real project

Manual Testing Details

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• IDE used for testing:
• Project(s) tested against:
• Specific features/scenarios verified:

Checklist

• My code follows the existing code style and conventions
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have updated BUILD.bazel files if I added new source files
• My changes generate no new warnings
• Any dependent changes have been merged and published

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Related Issues

Fixes #(issue number)
Closes #(issue number)
Related to #(issue number)

Additional Notes

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