Add support for default exports of anonymous functions and classes

[petermasking]

Currently, default-exported functions and classes must have an identifier because our FQN generation includes the declaration name.

// path/to/function.ts
export default function run() {} // FQN: path/to/run

// path/to/class.ts
export default class Thing {} // FQN: path/to/Thing

With barrel file support, however, the FQN can now be derived entirely from the resolved export path.

// path/to/run/index.ts
export default function execute() {} // FQN: path/to/run

In this case, the declaration name is ignored, making it unnecessary overhead. We should therefore also support anonymous default exports.

// path/to/run/index.ts
export default function() {} // FQN: path/to/run