Make topmod's name the empty string in rustdoc

[catamorphism]

From rustdoc::doc:

// FIXME: We currently give topmod the name of the crate.  There would
// probably be fewer special cases if the crate had its own name and
// topmod's name was the empty string.

[emberian]

Done with rustdoc_ng... kinda. The frontend does this. The JSON's only name is the name of the "topmod".