When generating completions for Cargo, if the Rust sysroot
(i.e. `rustc +${${(z)$(rustup default)}[1]} --print sysroot`) contains
spaces, Cargo completions will not work because the spaces are not
escaped, thus passing two arguments to the "source" command instead of
one. The spaces need to be escaped for this to work.

By default, `typeset` defines variables locally unless in the main scope.
This is specially bad when using `omz plugin load`, which happens inside
a function, so the declared variables don't continue being defined when
the function finishes and the main scope reappears.

Fixes #10562

Fixes #10560

Also cleaned up the code a bit

Fixes #10561