Microsoft.DotNet.ApiCompat.Tool CP0001 mis-fires across target frameworks when type is replaced with type forwarder

### Describe the bug

When a type is declared for netstandard2.0 and replaced with a type forwarder in netstandard2.1, CP0001 claims we have an API compatibility error between the two dll's.

### To Reproduce

```cs
#if NETSTANDARD2_0
public class System.Buffers.SequenceReader(ReadOnlySequence sequence) { }
#else
[assembly: TypeForwardedTo(typeof(System.Buffers.SequenceReader<>))]
#endif
```

Expected: No errors (assuming the forwarded to `SequenceReader` is in fact API compatible with the original ns2.0 declaration)

Actual:

```
CP0001: Type 'System.Buffers.SequenceReader' exists on lib/netstandard2.0/Nerdbank.Streams.dll but not on lib/netstandard2.1/Nerdbank.Streams.dll
```

### Further technical details

microsoft.dotnet.apicompat.tool 9.0.304

Start by reproducing the CP0001 report with the conditional C# example and microsoft.dotnet.apicompat.tool 9.0.304. Read the Microsoft.DotNet.ApiCompat.Tool handling for type declarations and type forwarders across target frameworks. Done means the netstandard2.0 declaration and netstandard2.1 forwarder are treated as compatible when the forwarded type is API compatible.

Written by the indexing model from the issue text.