environment: make sure multiple-name program lookup is always equal

We support this in a machine file:

```
[binaries]
pkgconfig = 'pkg-config'
pkg-config = 'pkg-config'
```

and you can use either one, because internally we look up both. But this
plays awkwardly with setting $PKG_CONFIG, since we don't know which one
you set in the machine file and the *other* one will be initialized from
the environment instead.

We can't solve this by changing the order of lookup, because we need to
know that we checked for machine file entries before falling back to the
environment. Instead, make one compatible machine file entry propagate
to the other possibilities before we finish populating the binary table
with environment variables.

mesonbuild/environment.py

@@ -585,6 +585,8 @@ def __init__(self, source_dir: T.Optional[str], build_dir: T.Optional[str], opti
         # Command line options override those from cross/native files
         self.options.update(options.cmd_line_options)
 
+        # Normalize inconsistent legacy binary names before falling back to env.
+        self._match_remapped_names()
         # Take default value from env if not set in cross/native files or command line.
         self._set_default_options_from_env()
         self._set_default_binaries_from_env()
@@ -734,6 +736,21 @@ def _set_default_options_from_env(self) -> None:
             if k not in self.options:
                 self.options[k] = v
 
+    def _match_remapped_names(self) -> None:
+        """Some binaries can appear in a machine file under multiple names."""
+
+        bins = [('pkg-config', 'pkgconfig')]
+        for binset, for_machine in itertools.product(bins, MachineChoice):
+            canonical: T.Optional[T.List[str]] = None
+            for name in binset:
+                hit = self.lookup_binary_entry(for_machine, name)
+                if hit is not None:
+                    canonical = hit
+                    break
+            if canonical:
+                for name in binset:
+                    self.binaries[for_machine].binaries.setdefault(name, canonical)
+
     def _set_default_binaries_from_env(self) -> None:
         """Set default binaries from the environment.