Use TypeSystem.String instead of typeof(string) when resolving types from Cecil

[maxkatz6]

How was the solution implemented (if it's not obvious)?

1. typeof(string) returns RuntimeType defined in System.Private.CoreLib.
2. System.Private.CoreLib is used from the build runtime version, i.e. 7.0.
3. It's getting included in the built assembly with the same version.
4. Avoiding typeof helps to avoid System.Private.CoreLib completely.

TIL, to not use typeof with cecil.

Fixed issues

Fixes #10859

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0034696-beta. (feed url: https://pkgs.dev.azure.com/AvaloniaUI/AvaloniaUI/_packaging/avalonia-all/nuget/v3/index.json) [PRBUILDID]

[robloo]

This is a really good find! Still not sure how typeof(string) is resolving the way it is though. Very interesting

[maxkatz6]

@robloo in short, Cecil used runtime reflection to resolve and import type instead of looking into assemblies known at compile time. Quite a known problem there apparently jbevain/cecil#895