Don't parse files of elem and mods

[blond]

Example:

button/
  __text/
    button__text.css
    button__text.js

We can only test that stem (basename without extension) of the button__text.css file matches on button dirname.

const stem = getStem('button__text.css');

if (stem === 'button__text') {
    // add button__text
}

Now we still parse stem:

• https://github.com/bem-sdk/bem-walk/blob/master/lib/walkers/nested.js#L167
• https://github.com/bem-sdk/bem-walk/blob/master/lib/walkers/nested.js#L134
• https://github.com/bem-sdk/bem-walk/blob/master/lib/walkers/nested.js#L196

For modifiers you cannot use only stem because of key-value modifiers:

button/
  _size/
    button__size_m.css
    button__size_l.css