function match (name, version, options = {}) {
	const escapedName = escapeName(name)
	const {registry = DEFAULT_REGISTRY, ...fetchOptions} = options
	const uri = url.resolve(registry, `${escapedName}/${version}`)
	return fetch(uri, fetchOptions)::map(({body}) => body)
}
resolve a package defined via an ambiguous semantic version string to a
specific `package.json` file.
@param {String} name - package name.
@param {String} version - semantic version string or tag name.
@param {Object} options - HTTP request options.
@return {Observable} - observable sequence of the `package.json` file.