Part nested into pseudo selector

This example work fine:

``` scss
@include component(foo) {
& {
@include part(bar) {
// something
}
}
}
```

``` css
.foo__bar {
padding: 1em;
}
```

This produces a wrong output:

``` scss
@include component(foo) {
&:last-child {
@include part(bar) {
// something
}
}
}
```

``` css
/* Expected */
.foo:last-child .foo__bar {
padding: 1em;
}

/* Given */
.foo:last-child__bar {
padding: 1em;
}
```

Using **:not** pseudo-element will produce an error:

``` scss
@include component(foo) {
&:not(:first-child) {
@include part(bar) {
// something
}
}
}
```

The thrown error is `invalid selector after )`

Reproduce the nested `&:last-child` and `&:not(:first-child)` examples from the issue, then trace the selector-nesting entry point that produces `__bar` or `invalid selector after )`. Done means pseudo selectors remain on `.foo` while the part selector becomes `.foo__bar`, with no parsing error.

Written by the indexing model from the issue text.