This requires moving resolveSymlinks() into SourceAccessor. Also, it
requires LocalStoreAccessor::maybeLstat() to work on parents of the
store (to avoid an error like "/nix is not in the store").
Fixes#10375.
As discussed in the last Nix team meeting (2024-02-95), this method
doesn't belong because `CanonPath` is a virtual/ideal absolute path
format, not used in file systems beyond the native OS format for which a
"current working directory" is defined.
Progress towards #9205
The code has already been fixed (yay!) so what is left of this commit is
just updating the API docs.
Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>