Fixed "::::" appearing in module_path!()
Added test for module_path! fix
auto merge of #19262 : murarth/rust/module-path-fix, r=jakub-

Closes #18859
Update guide.md intro and Installing Rust

Made things a tad less informal, broke up Installation section with some subheaders, and adjusted some wording.

Work on rust-lang/rust#19335.