Emit warning/error when System.Memory reference is missing but required by emitted code

Actual behavior

As soon as I add 'CharLower' to my NativeMethods.txt I get the following build error:

Error	CS0246	The type or namespace name 'Span<>' could not be found (are you missing a using directive or an assembly reference?)
C:\dev\_tests\ConsoleApp3\Microsoft.Windows.CsWin32\Microsoft.Windows.CsWin32.SourceGenerator\PWSTR.g.cs

Other methods like 'MFStartup' which don't take strings work fine.

Expected behavior

The project should compile without errors even if using .NET Framework v4.8 / netstandard2.0

Repro steps

1. Create new .NET Framework 4.8 project or create .NET 5.0 project and change target framework to 'net48' or 'netstandard2.0'.

2. NativeMethods.txt content:

CharLower

3. NativeMethods.json content (if present): does not exist

4. Build project

Context

• CsWin32 version: 0.1.422-beta
• Win32Metadata version (if explicitly set by project): default
• Target Framework: net48 or netstandard2.0
• LangVersion: 8.0

Reproduce the issue with NativeMethods.txt containing CharLower while targeting net48 or netstandard2.0, then inspect the generated PWSTR.g.cs and the source generator's dependency handling. Compare the build output with and without a System.Memory reference. Done means the missing requirement is reported clearly or the generated project builds without the opaque Span<> CS0246 error.