Warn on duplicate LLSD keys in autobuild package definitions

[Copilot]

Autobuild silently accepts duplicate LLSD map keys in package metadata, including repeated 3P/platform definitions introduced by merges. This change surfaces those conflicts at load time without changing current resolution behavior.

• Behavior change

  • Detect duplicate LLSD keys while reading autobuild.xml and metadata files.
  • Log a warning with the full key path so conflicting entries are easy to locate.
  • Preserve existing semantics: later entries still override earlier ones.

• Scope

  • Covers duplicate installable names, duplicate platform blocks, and any other repeated LLSD map keys in nested package data.
  • Applies before llsd.parse() collapses duplicates into a single dict entry.

• Regression coverage

  • Add a focused configfile test for a duplicated platform entry under one installable.
  • Assert both the emitted warning and the retained last-definition-wins behavior.

• Example

  <key>installables</key>
  <map>
    <key>icu4c</key>
    <map>
      <key>platforms</key>
      <map>
        <key>linux64</key>
        <map>...</map>
        <key>linux64</key>
        <map>...</map>
      </map>
    </map>
  </map>

  This now warns on:

  installables.icu4c.platforms.linux64
  

• Fixes autobuild happily accepts multiple declarations of the same 3P. #44

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[mcgroarty]

@copilot Document new functions and any changes that would not be clear to a mid-level engineer or an engineer who is not already familiar with AutoBuild

[mcgroarty]

I have read the CLA Document and I hereby sign the CLA

[Copilot]

@copilot Document new functions and any changes that would not be clear to a mid-level engineer or an engineer who is not already familiar with AutoBuild

Addressed in d5126a6. I added docstrings for the duplicate-key helper functions and an inline note at the load site explaining why the warning has to run on raw LLSD XML before parsing collapses duplicate keys.

[mcgroarty]

The windows-latest test failure is unrelated to this branch. That's fixed in #71