Expose synchronous SymbolFinder.FindSourceDefinition

[CyrusNajmabadi]

The existing api is actually implemented this way:

    /// <summary>
    /// Finds the definition symbol declared in source code for a corresponding reference symbol. 
    /// Returns null if no such symbol can be found in the specified solution.
    /// </summary>
    public static Task<ISymbol?> FindSourceDefinitionAsync(ISymbol? symbol, Solution solution, CancellationToken cancellationToken = default)
        => Task.FromResult(FindSourceDefinition(symbol, solution, cancellationToken));

    /// <inheritdoc cref="FindSourceDefinitionAsync"/>
    internal static ISymbol? FindSourceDefinition(
        ISymbol? symbol, Solution solution, CancellationToken cancellationToken)

Where the public async entrypoint actually just defers to the synchronous impl. This is more painful to use for code that exists in layers that are public only, but which are in synchronous codepaths (for example, in syntax rewriters).

We shoudl just expose the synchronous version and optionally obsolete the async version, recommending people use it instead.