Console spam when parsing inline sourcemaps

# Problem
When this addon encounters an inline sourcemap (`sourceMappingURL=data:application/json;charset=utf-8;base64,[giant base64 blob...]`), it fails to parse it and generates a huge console message containing the entire base64 blob.

# Cause
The `source-map-url` package used in this lib will return the entire inline sourcemap when you call `getFrom(code)`.

This becomes an issue in `get-sourcemap-content.js`, because it always assumes `getFrom(code)` returns a url.

The filesystem exists check fails, and a `console.warn` message fires off saying the `url` referenced could not be found. However this `url` is actually the huge inline sourcemap blob!