ExternalProgram: add full_command to complete the offfering

In case of python and especially in the case of pyInstaller
where the python command is meson.exe runpython, it should
not be full path to be used but full_command.

Fixing #13834

mesonbuild/interpreter/interpreterobjects.py

@@ -618,7 +618,8 @@ def __init__(self, ep: _EXTPROG, interpreter: 'Interpreter') -> None:
         self.methods.update({'found': self.found_method,
                              'path': self.path_method,
                              'version': self.version_method,
-                             'full_path': self.full_path_method})
+                             'full_path': self.full_path_method,
+                             'full_command': self.full_command_method })
 
     @noPosargs
     @noKwargs
@@ -645,6 +646,19 @@ def _full_path(self) -> str:
         assert path is not None
         return path
 
+    @noPosargs
+    @noKwargs
+    @FeatureNew('ExternalProgram.full_command', '1.6.1')
+    def full_command_method(self, args: T.List[TYPE_var], kwargs: TYPE_kwargs) -> str:
+        return self._full_command()
+
+    def _full_command(self) -> str:
+        if not self.found():
+            raise InterpreterException('Unable to get the path of a not-found external program')
+        path = self.held_object.get_command()
+        assert path is not None
+        return path
+
     @noPosargs
     @noKwargs
     @FeatureNew('ExternalProgram.version', '0.62.0')