function isRelativeSourcePath(source) {
  return source.startsWith('.');
}
Determines for a source path of an import- or export specifier, whether
it is relative (an internal import/export) or absolute (external)
- relative: './helpers', './helpers.js', '../helpers.js'
- not relative: '@open-wc/helpers', 'project-x/helpers'
@param {string} source source path of an import- or export specifier
@returns {boolean}