* Expand path when converting value -> PathBuf
Also includes Tagged<PathBuf>.
Fixes#3605
* Expand path for PATH env. variable
Fixes#1834
* Remove leftover Cows after nu-path refactor
There were some unnecessary Cow conversions leftover from the old
nu-path implementation.
* Use canonicalize in source command; Improve errors
Previously, `source` used `expand_path()` which does not follow
symlinks.
As a follow up, I improved the source error messages so they now tell
why the source file could not be canonicalized or read into string.