Make API Compat check for cases where compiler emits references to internal modreq's in methods

[ericstj]

See dotnet/runtime#61737

In this case the compiler emits references to the modreq [System.Text.Json]System.Runtime.CompilerServices.IsExternalInit. This type is defined as internal in System.Text.Json. If this appears in a public method, then we need to treat the modreq-target-type as effectively public, even if it is internal.

Treating it as public will ensure it is defined or forwarded in compatible assemblies.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ericstj]

cc @andschwa who's been dealing with this issue in OmniSharp and PowerShell.

We noticed that a lot of folks have made this mistake, https://github.com/search?q=%22static+class+IsExternalInit%22+-file%3A*tests&type=Code

Might be a good idea to prioritize this rule in 8.0.