Preprocessors using `pulldown-cmark-to-cmark` do not yet perform round
trips 100% correctly, and insert leading spaces and an extra initial `>`
for block quotes, so strip those. Once that is fixed upstream, this will
become a no-op, and can be removed then.
Rust recently began printing error messages which include the full path
to the source of the file with an error in it; when that path is in the
standard library, that includes the user's home directory (or similar).
A previous change (b9241f6) added support for those paths in the output
files; this one adds support to the local file paths linter.
Specifically allow exactly and only paths in the home directory which
include `.rustup` or `.cargo`.
In addition to the baseline changes, skip over non-directory code where
directories are needed to deal with things like `.DS_Store` files. Also
add a bunch of context on error causes from `std::io::Error` because it
was *impossible* to figure out exactly what the source of those were.
