private int findOffsetFromStart(String location) {
        // This accounts for "jar:file:" and "file:".
        int position = location.indexOf("file:");
        return position == -1 ? 0 : position + "file:".length();
    }
Checks if the given metadata key-value pair is either not present
in the old model (thus a new, source metadata), or is present and
the filename of the entry in the original model matches one of
the defined {@code sources}.

@param metadataKey Metadata key to check.
@return Returns true if this metadata is considered a source entry.
smithy-build/src/main/java/software/amazon/smithy/build/PluginContext.java