Rename the rsvg package to librsvg

@sophie-h @BrainBlasted @sdroege Is this the sort of renaming you had in mind? TL;DR - in rsvg/Cargo.toml:

[package]
name = "librsvg"

[lib]
name = "rsvg"

(This is different from the toplevel Cargo.toml which just holds the workspace information.)

The code still uses rsvg::Loader and whatnot.

Aside: https://crates.io/crates/librsvg exists already, and it is not mine. Do you know who owns it?