Warn about 2015 edition on path & extern crate related errors #63323
Description
Users may be using 2015 edition without being aware of it, e.g. by copying an older template/tutorial or creating Cargo.toml by hand.
Accidental use of 2015 edition may cause puzzling path-related errors, such as E0432 & E0433.
When 2015-edition code fails to resolve paths, rustc could say if it works in 2018 edition (#61914) or at least emit a note saying that the failing code uses 2015 edition, and that switching to 2018 may help.