Package URL encoding should be dealt correctly

```
>>> from fetchcode import package
>>> list(package.info('pkg:npm/@babel/babel-runtime'))
Traceback (most recent call last):
File "", line 1, in
File "w421/fetchcode/src/fetchcode/package.py", line 117, in get_npm_data_from_purl
purl = PackageURL.from_string(purl)
File "w421/fetchcode/tmp/lib/python3.6/site-packages/packageurl/__init__.py", line 408, in from_string
'name component: {}'.format(repr(purl)))
ValueError: purl is missing the required name component: 'pkg:npm/@babel/babel-runtime'

```
fails...
but IMHO it should work
this works:
```
list(package.info('pkg:npm/%40babel/babel-runtime'))
```