[SCons] Improve staged installations of Python module

Fixes setup_cantera and the post-install message to reference the path
of the Python module on the target system rather than the staging
directory.

Also, generated .pyc files no long specify the path in the staging
directory. This has no effect on the use of the .pyc files, but helps
avoid warnings/errors from packaging system linters (notably, on
FreeBSD).

Fixes #1094

interfaces/cython/SConscript

@@ -2,6 +2,7 @@
 import re
 from os.path import join as pjoin
 from os.path import normpath
+from pathlib import Path
 from pkg_resources import parse_version
 from buildutils import *
 
@@ -122,6 +123,14 @@ else:
     # Install Python module in the default location
     extra = ''
 
+if env["stage_dir"]:
+    # Get the absolute path to the stage directory. If the stage directory is a relative
+    # path, consider it to be relative to the root of the Cantera source directory,
+    # which is two directories up from the current working directory of this SConscript
+    # file.
+    stage_absolute = Path.cwd().parents[1].joinpath(env['stage_dir']).resolve()
+    extra += f" --root={stage_absolute}"
+
 env['python_module_loc'] = '<unspecified>'
 if localenv['PYTHON_INSTALLER'] == 'direct':
     mod_inst = install(localenv.Command, 'dummy', mod,
@@ -134,9 +143,9 @@ if localenv['PYTHON_INSTALLER'] == 'direct':
         for filename in open('build/python-installed-files.txt').readlines():
             filename = filename.strip()
             if filename.endswith(check):
-                filename = filename.replace(check, '')
-                global_env['python_module_loc'] = normpath(filename)
+                global_env["python_module_loc"] = normpath(filename.replace(check, ""))
                 break
+
     localenv.AlwaysBuild(localenv.AddPostAction(mod_inst, find_module_dir))
     env['install_python_action'] = mod_inst
 elif localenv['PYTHON_INSTALLER'] == 'debian':