Bug 1636574 - Allow setting `PYTHON3` in mozconfig r=glandium

Configuration values defined above `mozconfig_options` in `init.configure` are those that can't be configured in `mozconfig`. As far as I can tell there is nothing wrong *in principle* with setting `PYTHON3` in `mozconfig`, so here we just bump `mozconfig_options` above `PYTHON3` configuration.

Side note, diagnosing this failure took a *long* time. The error message that this produced ("unknown option `PYTHON3`") is useless and the underlying algos being extremely mutable, I ended up having to spend a lot of time in the debugger and monitor all the underlying changes to a bunch of mutable data structures to determine where `PYTHON3` was getting lost. A better error message would be good here but I don't know how I would begin adding it.

Differential Revision: https://phabricator.services.mozilla.com/D75635

build/moz.configure/init.configure

@@ -233,6 +233,52 @@ def help_shell(help, shell):
 shell = help_shell | shell
 
 
+# Inject mozconfig options
+# ==============================================================
+
+
+@depends(mozconfig, 'MOZ_AUTOMATION', '--help')
+# This gives access to the sandbox. Don't copy this blindly.
+@imports('__sandbox__')
+@imports('os')
+@imports('six')
+def mozconfig_options(mozconfig, automation, help):
+    if mozconfig['path']:
+        if 'MOZ_AUTOMATION_MOZCONFIG' in mozconfig['env']['added']:
+            if not automation:
+                log.error('%s directly or indirectly includes an in-tree '
+                          'mozconfig.', mozconfig['path'])
+                log.error('In-tree mozconfigs make strong assumptions about '
+                          'and are only meant to be used by Mozilla '
+                          'automation.')
+                die("Please don't use them.")
+        helper = __sandbox__._helper
+        log.info('Adding configure options from %s' % mozconfig['path'])
+        for arg in mozconfig['configure_args']:
+            log.info('  %s' % arg)
+            # We could be using imply_option() here, but it has other
+            # contraints that don't really apply to the command-line
+            # emulation that mozconfig provides.
+            helper.add(arg, origin='mozconfig', args=helper._args)
+
+        def add(key, value):
+            if key.isupper():
+                arg = '%s=%s' % (key, value)
+                log.info('  %s' % arg)
+                helper.add(arg, origin='mozconfig', args=helper._args)
+
+        for key, value in six.iteritems(mozconfig['env']['added']):
+            add(key, value)
+            os.environ[key] = value
+        for key, (_, value) in six.iteritems(mozconfig['env']['modified']):
+            add(key, value)
+            os.environ[key] = value
+        for key, value in six.iteritems(mozconfig['vars']['added']):
+            add(key, value)
+        for key, (_, value) in six.iteritems(mozconfig['vars']['modified']):
+            add(key, value)
+
+
 # Python 3
 # ========
 
@@ -397,71 +443,6 @@ set_config('PYTHON3', virtualenv_python3.path)
 set_config('PYTHON3_VERSION', virtualenv_python3.str_version)
 
 
-# Inject mozconfig options
-# ==============================================================
-# All options defined above this point can't be injected in mozconfig_options
-# below, so collect them.
-
-
-@template
-def early_options():
-    @dependable
-    @imports('__sandbox__')
-    @imports(_from='six', _import='itervalues')
-    def early_options():
-        return set(
-            option.env
-            for option in itervalues(__sandbox__._options)
-            if option.env
-        )
-    return early_options
-
-
-early_options = early_options()
-
-
-@depends(mozconfig, 'MOZ_AUTOMATION', '--help')
-# This gives access to the sandbox. Don't copy this blindly.
-@imports('__sandbox__')
-@imports('os')
-@imports('six')
-def mozconfig_options(mozconfig, automation, help):
-    if mozconfig['path']:
-        if 'MOZ_AUTOMATION_MOZCONFIG' in mozconfig['env']['added']:
-            if not automation:
-                log.error('%s directly or indirectly includes an in-tree '
-                          'mozconfig.', mozconfig['path'])
-                log.error('In-tree mozconfigs make strong assumptions about '
-                          'and are only meant to be used by Mozilla '
-                          'automation.')
-                die("Please don't use them.")
-        helper = __sandbox__._helper
-        log.info('Adding configure options from %s' % mozconfig['path'])
-        for arg in mozconfig['configure_args']:
-            log.info('  %s' % arg)
-            # We could be using imply_option() here, but it has other
-            # contraints that don't really apply to the command-line
-            # emulation that mozconfig provides.
-            helper.add(arg, origin='mozconfig', args=helper._args)
-
-        def add(key, value):
-            if key.isupper():
-                arg = '%s=%s' % (key, value)
-                log.info('  %s' % arg)
-                helper.add(arg, origin='mozconfig', args=helper._args)
-
-        for key, value in six.iteritems(mozconfig['env']['added']):
-            add(key, value)
-            os.environ[key] = value
-        for key, (_, value) in six.iteritems(mozconfig['env']['modified']):
-            add(key, value)
-            os.environ[key] = value
-        for key, value in six.iteritems(mozconfig['vars']['added']):
-            add(key, value)
-        for key, (_, value) in six.iteritems(mozconfig['vars']['modified']):
-            add(key, value)
-
-
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