which: add external find support (#30056)

Co-authored-by: Robert Underwood <runderwood@anl.gov>

var/spack/repos/builtin/packages/which/package.py

@@ -2,6 +2,8 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
+import re
+
 from spack import *
 
 
@@ -13,3 +15,11 @@ class Which(AutotoolsPackage):
     url      = "https://ftp.gnu.org/gnu/which/which-2.21.tar.gz"
 
     version('2.21', sha256='f4a245b94124b377d8b49646bf421f9155d36aa7614b6ebf83705d3ffc76eaad')
+
+    executables = ['which']
+
+    @classmethod
+    def determine_version(cls, exe):
+        output = Executable(exe)('--version', output=str, error=str)
+        match = re.search(r'GNU which v(\d+.\d+)', output)
+        return match.group(1) if match else None