Bug 1670167 - Fix janky Python 3 logic in configure from bug 1656993 r=firefox-build-system-reviewers,glandium

The `try : import mozfile; except ImportError: ...` thing was completely broken since it unconditionally triggered [this exception](https://searchfox.org/mozilla-central/rev/165e8d8f80c02605c2f3e89c5de462abfce91c32/python/mozbuild/mozbuild/configure/__init__.py#248). The `MOZBUILD_VIRTUALENV` thing should work now, so just use that as a signal for whether we're already in the `virtualenv` instead.

Also delete some `PATH` munging in `building.py`, which I think should be unnecessary after we make the above change.

Differential Revision: https://phabricator.services.mozilla.com/D93138

build/moz.configure/init.configure

@@ -296,16 +296,13 @@ def virtualenv_python3(env_python, virtualenv_name, build_env, mozconfig, help):
 
     # If we're not in the virtualenv, we need to update the path to include some
     # necessary modules for find_program.
-    try:
-        import mozfile
-    except ImportError:
+    if 'MOZBUILD_VIRTUALENV' in os.environ:
+        python = sys.executable
+    else:
         sys.path.insert(
             0, os.path.join(topsrcdir, 'testing', 'mozbase', 'mozfile'))
         sys.path.insert(
             0, os.path.join(topsrcdir, 'third_party', 'python', 'backports'))
-    else:
-        if 'MOZBUILD_VIRTUALENV' in os.environ or not python:
-            python = sys.executable
 
     # If we know the Python executable the caller is asking for then verify its
     # version. If the caller did not ask for a specific executable then find

python/mozbuild/mozbuild/configure/__init__.py

@@ -292,7 +292,7 @@ class ConfigureSandbox(dict):
         for b in ('None', 'False', 'True', 'int', 'bool', 'any', 'all', 'len',
                   'list', 'tuple', 'set', 'dict', 'isinstance', 'getattr',
                   'hasattr', 'enumerate', 'range', 'zip', 'AssertionError',
-                  'ImportError', '__build_class__',  # will be None on py2
+                  '__build_class__',  # will be None on py2
                   )
     }, __import__=forbidden_import, str=six.text_type)
 

python/mozbuild/mozbuild/controller/building.py

@@ -1561,12 +1561,6 @@ class BuildDriver(MozbuildObject):
             }, 'Adding make options from {path}\n    {content}')
 
         append_env['OBJDIR'] = mozpath.normsep(self.topobjdir)
-        if (mozpath.normpath(os.path.dirname(sys.executable)) not in
-            [mozpath.normpath(s) for s in
-             os.environ['PATH'].split(os.pathsep)]):
-            append_env['PATH'] = (
-                os.path.dirname(sys.executable) + os.pathsep +
-                os.environ['PATH'])
 
         return self._run_make(srcdir=True,
                               filename='client.mk',