This changes preprocessors so that:
- Relative paths in the `command` value are relative to the book root.
- The process current directory is the book root.
This makes it so that it isn't dependent on the directory where `mdbook`
is executed.
Fixes https://github.com/rust-lang/mdBook/issues/1424
This updates everything for the move of utils to mdbook-core. There will
be followup commits that will be moving and refactoring these utils.
This simply moves them over unchanged (except visibility).
This moves Result and Error to mdbook-core with the anticipation of
using them in user crates. For now, the internal APIs will be using
anyhow directly, but the intent is to transition more of these to
mdbook-core where it makes sense.
