clean up get_env_var_pair

This function returns both the name and the value, but we never actually
use the name, just the value. Also make this module private. We really
want to keep all environment variable reading in the Environment class
so it's done once up front. This should help with that goal.

mesonbuild/environment.py

@@ -142,9 +142,7 @@ if T.TYPE_CHECKING:
     import argparse
 
 
-def get_env_var_pair(for_machine: MachineChoice,
-                     is_cross: bool,
-                     var_name: str) -> T.Optional[T.Tuple[str, str]]:
+def _get_env_var(for_machine: MachineChoice, is_cross: bool, var_name: str) -> T.Optional[str]:
     """
     Returns the exact env var and the value.
     """
@@ -166,7 +164,7 @@ def get_env_var_pair(for_machine: MachineChoice,
         mlog.debug('None of {} are defined in the environment, not changing global flags.'.format(formatted))
         return None
     mlog.debug('Using {!r} from environment with value: {!r}'.format(var, value))
-    return var, value
+    return value
 
 
 def detect_gcovr(min_version='3.3', new_rootdir_version='4.2', log=False):
@@ -791,10 +789,8 @@ class Environment:
         )
 
         for (evar, keyname), for_machine in itertools.product(opts, MachineChoice):
-            p_env_pair = get_env_var_pair(for_machine, self.is_cross_build(), evar)
-            if p_env_pair is not None:
-                _, p_env = p_env_pair
-
+            p_env = _get_env_var(for_machine, self.is_cross_build(), evar)
+            if p_env is not None:
                 # these may contain duplicates, which must be removed, else
                 # a duplicates-in-array-option warning arises.
                 if keyname == 'cmake_prefix_path':
@@ -843,9 +839,8 @@ class Environment:
                                envconfig.ENV_VAR_PROG_MAP.items())
 
         for (name, evar), for_machine in itertools.product(opts, MachineChoice):
-            p_env_pair = get_env_var_pair(for_machine, self.is_cross_build(), evar)
-            if p_env_pair is not None:
-                _, p_env = p_env_pair
+            p_env = _get_env_var(for_machine, self.is_cross_build(), evar)
+            if p_env is not None:
                 self.binaries[for_machine].binaries.setdefault(name, mesonlib.split_args(p_env))
 
     def _set_default_properties_from_env(self) -> None:
@@ -859,9 +854,8 @@ class Environment:
 
         for (name, evars, split), for_machine in itertools.product(opts, MachineChoice):
             for evar in evars:
-                p_env_pair = get_env_var_pair(for_machine, self.is_cross_build(), evar)
-                if p_env_pair is not None:
-                    _, p_env = p_env_pair
+                p_env = _get_env_var(for_machine, self.is_cross_build(), evar)
+                if p_env is not None:
                     if split:
                         self.properties[for_machine].properties.setdefault(name, p_env.split(os.pathsep))
                     else: