Add source provenance documentation to debian/README.Debian

[Copilot]

This PR implements source provenance documentation for Debian packages by automatically generating debian/README.Debian files that document where each source came from, similar to how RPM specs already include this information as comments.

Problem

RPM packages built with dalec include detailed source provenance documentation in the spec file comments, allowing consumers to understand and reproduce builds. However, Debian packages lacked this same functionality, making it difficult for users to understand the origins of package sources.

Solution

Added automatic generation of debian/README.Debian files during the Debian package build process. The implementation:

• Leverages existing infrastructure: Uses the existing Source.Doc() method that's already implemented for all source types
• Provides comprehensive documentation: Supports all source types including git repositories, HTTP archives, Docker images, and inline content
• Follows Debian conventions: Places documentation in the standard debian/README.Debian location as recommended by Debian packaging guidelines
• Maintains consistency: Provides similar information to what's already available in RPM specs
• Optimizes for packages with sources: Only creates README.Debian files when sources are present, avoiding unnecessary files for packages without sources

Example Output

For a package with multiple source types, the generated debian/README.Debian looks like:

myapp for Debian
================

My Application - A sample application built with dalec

Source Provenance
-----------------

This package was built from the following sources:

Source: config.yaml
  Generated from an inline source:
  cat << EOF > config.yaml
  server:
    port: 8080
  EOF
  chmod 644 config.yaml

Source: myapp
  Generated from a git repository:
  Remote: https://github.com/example/myapp.git
  Ref: v1.0.0

Source: vendor-deps.tar.gz
  Generated from a http(s) source:
  URL: https://cdn.example.com/deps/myapp-deps-1.0.0.tar.gz

Implementation Details

• Added generateReadmeDebian() function in packaging/linux/deb/debroot.go
• Integrated README.Debian generation into the existing Debroot() function
• Added comprehensive test coverage for various source types and edge cases
• Maintains alphabetical source ordering for consistent output
• Returns nil when no sources are present to avoid creating empty documentation files
• Zero breaking changes to existing functionality

Fixes #556.

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[cpuguy83]

@copilot Why do we need an exported function?

[cpuguy83]

@copilot this should return before we've written anything and just return a nil slice.

[Copilot]

Fixed in fed46d7. The function now returns nil immediately when there are no sources, avoiding creation of README.Debian file entirely. Updated calling code to handle nil return value and modified tests accordingly.

[cpuguy83]

@copilot rebase

[cpuguy83]

@copilot The import path is now github.com/project-dalec/dalec