Generate pdb for ref assemblies

[MrJul]

External symbols, pass 3

What is the current behavior?

Since #12216, pdb files for lib assemblies are external.

However, ref assemblies weren't modified:

• ✔️ Stepping into code works for packages without ref assemblies (e.g. Avalonia.Skia).
• ✔️ Stepping into code works for packages with ref assemblies (Avalonia main package) since lib assemblies are always involved when running/debugging.
• ✔️ Navigating to definition works for packages without ref assemblies.
• ❌ Navigating to definition doesn't work for package with ref assemblies.

The current ref assembly generator still embeds a pdb, but it's empty.
Visual Studio doesn't even fallback to the built-in decompiler in this case. Rider does.

What does the pull request do?

The GenerateRefAsmsInPackage has been rewritten to also patch the pdb files in the symbols package.

What is the updated/expected behavior with this PR?

Stepping into and navigating to definition work for all Avalonia packages in both Rider and Visual Studio.

Fixed issues

Fixes #12407

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0038275-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]