var controllers =
context.Compilation
    .SyntaxTrees
    .SelectMany(syntaxTree => syntaxTree.GetRoot().DescendantNodes())
    .Where(x => x is ClassDeclarationSyntax)
    .Cast<ClassDeclarationSyntax>()
    .Where(c => c.Identifier.ValueText.EndsWith("Controller", StringComparison.OrdinalIgnoreCase))
    .ToImmutableList();

public class ControllerFinder : ISyntaxReceiver
{
    public List<ClassDeclarationSyntax> Controllers { get; }
        = new();
    
    public void OnVisitSyntaxNode(SyntaxNode syntaxNode)
    {
        if (syntaxNode is ClassDeclarationSyntax controller)
        {
            if (controller.Identifier.ValueText.EndsWith("Controller"))
            {
                Controllers.Add(controller);
            }
        }
    }
}

public void Initialize(GeneratorInitializationContext context)
{
    context.RegisterForSyntaxNotifications(() => new ControllerFinder());
}

public void Execute(GeneratorExecutionContext context)
{
    var controllers = 
        ((ControllerFinder) context.SyntaxReceiver)?.Controllers;
    
    // use controllers to do work...
}

<PropertyGroup>
   <TargetFramework>netstandard2.0</TargetFramework>
   <IsRoslynComponent>true</IsRoslynComponent>

#if DEBUG
if (!Debugger.IsAttached)
{
   Debugger.Launch();
}
#endif