Rewrite canonicalize_path from scratch

The current implementation of canonicalize_path relies on system APIs
(std::filesystem::canonical or realpath). This works well, but I want to
customize how canonicalization works in order to support finding config
files for LSP documents not saved on disk. In particular, I want
something like std::filesystem::weakly_canonical, but there is no
equivalent in glibc (Linux) or libSystem (macOS).

Implement our own version of canonicalize_path from mostly first
principles. This will let us tweak the behavior in the future.

Author: strager