Auto merge of #54861 - rep-nop:find_main_in_doctest, r=estebank

rustdoc: Replaces fn main search and extern crate search with proper parsing during doctests.

Fixes #21299.
Fixes #33731.

Let me know if there's any additional changes you'd like made!

diff --cc src/libsyntax/parse/lexer/mod.rs
Simple merge

diff --cc src/libsyntax/parse/mod.rs
Simple merge