function normalizeBin (pkgJson) {
	switch (typeof pkgJson.bin) {
		case 'string': return ({[pkgJson.name]: pkgJson.bin})
		case 'object': return pkgJson.bin
		default: return {}
	}
}
normalize the `bin` property in `package.json`, which could either be a
string, object or undefined.
@param  {Object} pkgJson - plain JavaScript object representing a
`package.json` file.
@return {Object} - normalized `bin` property.