Bugs in pdflatex (#12804) are preventing the guide from landing (#16657). This solves the immediate problem by changing the build system to prefer lualatex, xelatex to pdflatex (which is apparently obsolete). Because the xelatex on the snapshot bot seems to completely ignore the `-output-directory` option, I also had to frob the makefiles a bit for that case.

handle {self} when removing unused imports

Fixes rust-lang#17139

On master

```rs
mod inner {
    pub struct X();
    pub struct Y();
}

mod z {
    use super::inner::{self, X}$0;

    fn f() {
        let y = inner::Y();
    }
}
```

becomes

```rs
mod inner {
    pub struct X();
    pub struct Y();
}

mod z {
    use super::inner:self;

    fn f() {
        let y = inner::Y();
    }
}
```

with this fix it instead becomes

```

```rs
mod inner {
    pub struct X();
    pub struct Y();
}

mod z {
    use super::inner;

    fn f() {
        let y = inner::Y();
    }
}
```