os/exec: return error when PATH lookup would use current directory

Following discussion on #43724, change os/exec to take the
approach of golang.org/x/sys/execabs, refusing to respect
path entries mentioning relative paths by default.

Code that insists on being able to find executables in relative
directories in the path will need to add a couple lines to override the error.

See the updated package docs in exec.go for more details.

Fixes #43724.
Fixes #43947.

Change-Id: I73c1214f322b60b4167a23e956e933d50470fe13
Reviewed-on: https://go-review.googlesource.com/c/go/+/381374
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>

src/os/exec/lp_unix.go

@@ -31,7 +31,11 @@ func findExecutable(file string) error {
 // LookPath searches for an executable named file in the
 // directories named by the PATH environment variable.
 // If file contains a slash, it is tried directly and the PATH is not consulted.
-// The result may be an absolute path or a path relative to the current directory.
+// Otherwise, on success, the result is an absolute path.
+//
+// In older versions of Go, LookPath could return a path relative to the current directory.
+// As of Go 1.19, LookPath will instead return that path along with an error satisfying
+// errors.Is(err, ErrDot). See the package documentation for more details.
 func LookPath(file string) (string, error) {
 	// NOTE(rsc): I wish we could use the Plan 9 behavior here
 	// (only bypass the path if file begins with / or ./ or ../)
@@ -52,6 +56,9 @@ func LookPath(file string) (string, error) {
 		}
 		path := filepath.Join(dir, file)
 		if err := findExecutable(path); err == nil {
+			if !filepath.IsAbs(path) {
+				return path, &Error{file, ErrDot}
+			}
 			return path, nil
 		}
 	}