pkgconfig: Allow setting both pkgconfig and pkg-config

This was previously allowed for different usage. Keep allowing it, but
with non fatal deprecation notice, and ignore the value from legacy
pkgconfig.

docs/markdown/snippets/pkgconfig_deprecated_machine_file.md

@@ -0,0 +1,10 @@
+## Machine files: `pkgconfig` field deprecated and replaced by `pkg-config`
+
+Meson used to allow both `pkgconfig` and `pkg-config` entries in machine files,
+the former was used for `dependency()` lookup and the latter was used as return
+value for `find_program('pkg-config')`.
+
+This inconsistency is now fixed by deprecating `pkgconfig` in favor of
+`pkg-config` which matches the name of the binary. For backward compatibility
+it is still allowed to define both with the same value, in that case no
+deprecation warning is printed.

mesonbuild/envconfig.py

@@ -404,12 +404,21 @@ def __init__(
                 if not isinstance(command, (list, str)):
                     raise mesonlib.MesonException(
                         f'Invalid type {command!r} for entry {name!r} in cross file')
-                if name == 'pkgconfig':
-                    if 'pkg-config' in binaries:
-                        raise mesonlib.MesonException('Both pkgconfig and pkg-config entries in machine file.')
-                    mlog.deprecation('"pkgconfig" entry is deprecated and should be replaced by "pkg-config"')
-                    name = 'pkg-config'
                 self.binaries[name] = mesonlib.listify(command)
+            if 'pkgconfig' in self.binaries:
+                if 'pkg-config' not in self.binaries:
+                    mlog.deprecation('"pkgconfig" entry is deprecated and should be replaced by "pkg-config"', fatal=False)
+                    self.binaries['pkg-config'] = self.binaries['pkgconfig']
+                elif self.binaries['pkgconfig'] != self.binaries['pkg-config']:
+                    raise mesonlib.MesonException('Mismatched pkgconfig and pkg-config binaries in the machine file.')
+                else:
+                    # Both are defined with the same value, this is allowed
+                    # for backward compatibility.
+                    # FIXME: We should still print deprecation warning if the
+                    # project targets Meson >= 1.3.0, but we have no way to know
+                    # that here.
+                    pass
+                del self.binaries['pkgconfig']
 
     @staticmethod
     def detect_ccache() -> T.List[str]: