Move from source to source-env (#6277)

* start working on source-env

* WIP

* Get most tests working, still one to go

* Fix file-relative paths; Report parser error

* Fix merge conflicts; Restore source as deprecated

* Tests: Use source-env; Remove redundant tests

* Fmt

* Respect hidden env vars

* Fix file-relative eval for source-env

* Add file-relative eval to "overlay use"

* Use FILE_PWD only in source-env and "overlay use"

* Ignore new tests for now

This will be another issue

* Throw an error if setting FILE_PWD manually

* Fix source-related test failures

* Fix nu-check to respect FILE_PWD

* Fix corrupted spans in source-env shell errors

* Fix up some references to old source

* Remove deprecation message

* Re-introduce deleted tests

Co-authored-by: kubouch <kubouch@gmail.com>

crates/nu-parser/src/parse_keywords.rs

@@ -3190,7 +3190,7 @@ pub fn parse_register(
 /// determined in the first step.
 ///
 /// Always returns an absolute path
-fn find_in_dirs(
+pub fn find_in_dirs(
     filename: &str,
     working_set: &StateWorkingSet,
     cwd: &str,