Finds a line in <full_doc> like

        <from_file_keyword> <colon> <path>

    and return path

def _find_from_file(full_doc, from_file_keyword):
    """
    Finds a line in <full_doc> like

        <from_file_keyword> <colon> <path>

    and return path
    """
    path = None

    for line in full_doc.splitlines():
        if from_file_keyword in line:
            parts = line.strip().split(':')
            if len(parts) == 2 and parts[0].strip() == from_file_keyword:
                path = parts[1].strip()
                break

    return path