Fix Cecil metadata importer

[maxkatz6]

What does the pull request do?

XamlX PR kekekeks/XamlX#88

Cecil default metadata importer seem to have not that much sense adding IL references from the dependencies of different version instead of using local ones. So, we need to help it find the way,
In general, it seems like MetadataImporter must be redefined if developer has changed AssemblyResolver logic, as these must be synced.
See submodule changes for more details kekekeks/XamlX@e5254eb...c5d5d8b

• Also updates Cecil version.
• Also replaces reflection importer with stub throwing exceptions, as we don't need reflection importer at all (see Use TypeSystem.String instead of typeof(string) when resolving types from Cecil #11363 for details).

What is the current behavior?

While building app in .NET 7 target with .NET 6 Avalonia (or .NET Core 2.1+ target with .NET Standard 2.0 Avalonia...).

Before Cecil+XAMLX:

After Cecil+XAMLX:

Duplicated references are from .NET 6.0 runtime (used in Avalonia assemblies APIs). But .NET 7.0 only is expected.

What is the updated/expected behavior with this PR?

Before Cecil+XAMLX: the same as it was.

After Cecil+XAMLX:

New IL references added, but all are of correct version used from target framework.

Fixed issues

Fixes #5456

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0034722-beta. (feed url: https://pkgs.dev.azure.com/AvaloniaUI/AvaloniaUI/_packaging/avalonia-all/nuget/v3/index.json) [PRBUILDID]