[BUG] Publishing with --access restricted should prevent packages with no namespace being pushed

[strongoose]

Current Behavior:

If a non-namespaced package is published with
npm publish --access restricted

npm ignored the user directive that access should be restricted and publishes the package.

Expected Behavior:

Attempting to publish a package which isn't namespaced with npm publish --access restricted should

1. warn the user that packages without namespaces cannot be given restricted access
2. exit with an error

Steps To Reproduce:

1. Create a non-namespaces NPM package
2. Publish is using npm publish --access restricted

[wraithgar]

Can't duplicate this in npm v7.

~/D/n/foo $ npm publish --access restricted
npm notice 
npm notice 📦  [email protected]
npm notice === Tarball Contents === 
npm notice 246B package.json
npm notice === Tarball Details === 
npm notice name:          foo                                     
npm notice version:       1.0.0                                   
npm notice filename:      foo-1.0.0.tgz                           
npm notice package size:  274 B                                   
npm notice unpacked size: 246 B                                   
npm notice shasum:        a1dd3e02b8f5d099099917dbdd0fd1a9418b79ef
npm notice integrity:     sha512-RRAjfWmDa3FX8[...]RLi+i/hCsWkJA==
npm notice total files:   1                                       
npm notice 
npm ERR! code EUNSCOPED
npm ERR! Can't restrict access to unscoped packages.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/wraithgar/.npm/_logs/2021-02-02T19_40_41_001Z-debug.log
~/D/n/foo $