feat(tvix/eval): coerce values to paths when importing

This enables the use of string paths (and, in the future,
derivations), as long as their string values represent an absolute
path.

Change-Id: I4b198efeb70415ed52f58bd1da6fa79a24dad14c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6866
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/errors.rs

@@ -262,7 +262,7 @@ to a missing value in the attribute set(s) included via `with`."#,
 
             ErrorKind::NotAnAbsolutePath(given) => {
                 format!(
-                    "string {} doesn't represent an absolute path",
+                    "string '{}' does not represent an absolute path",
                     given.to_string_lossy()
                 )
             }