Add missing MultiMethodMeta metaclass in builders (#45879)

* Add missing MultiMethodMeta metaclass in builders

and remove the Python 2 fallback option in favor of hard errors to catch
similar issues going forward.

The fallback option can cause about 10K stat calls due to use of
`realpath` in the inspect module, depending on how deep Spack itself is
nested in the file system, which is ... undesirable.

* code shuffling to avoid circular import

* more reshuffling

* move reserved variant names into variants module

1 files changed, 11 insertions, 9 deletions

diff --git a/var/spack/repos/builtin/packages/icu4c/package.py b/var/spack/repos/builtin/packages/icu4c/package.py
index 3cb4eb1d98..97f89cfc29 100644
--- a/var/spack/repos/builtin/packages/icu4c/package.py
+++ b/var/spack/repos/builtin/packages/icu4c/package.py
@@ -76,18 +76,15 @@ class Icu4c(AutotoolsPackage, MSBuildPackage):
         return (None, flags, None)
 
 
-class BuildEnvironment:
-    # Need to make sure that locale is UTF-8 in order to process source
-    # files in UTF-8.
+class AutotoolsBuilder(spack.build_systems.autotools.AutotoolsBuilder):
+
+    configure_directory = "source"
+
+    # Need to make sure that locale is UTF-8 in order to process source files in UTF-8.
     @when("@59:")
     def setup_build_environment(self, env):
         env.set("LC_ALL", "en_US.UTF-8")
 
-
-class AutotoolsBuilder(spack.build_systems.autotools.AutotoolsBuilder, BuildEnvironment):
-
-    configure_directory = "source"
-
     def configure_args(self):
         args = []
 
@@ -104,7 +101,12 @@ class AutotoolsBuilder(spack.build_systems.autotools.AutotoolsBuilder, BuildEnvi
         return args
 
 
-class MSBuildBuilder(spack.build_systems.msbuild.MSBuildBuilder, BuildEnvironment):
+class MSBuildBuilder(spack.build_systems.msbuild.MSBuildBuilder):
+    # Need to make sure that locale is UTF-8 in order to process source files in UTF-8.
+    @when("@59:")
+    def setup_build_environment(self, env):
+        env.set("LC_ALL", "en_US.UTF-8")
+
     def msbuild_args(self):
         return [
             "allinone.sln",