Add executable mx.py to repo root

Many places assume that there is an `mx.py` at the repo root that will
run mx.
The new mx.py is a stub that simply manipulates the path and then runs
the mx module

Author: patrick96

mx

@@ -76,11 +76,4 @@ if [ $? -ne 0 ]; then
     exit 1
 fi
 
-# Include the parent directory of this script in the python path variable, then
-# allow loading of the mx module
-export PYTHONPATH="$dir/src:$PYTHONPATH"
-
-# The following allows the use of existing module names
-export PYTHONPATH="$dir/oldnames:$PYTHONPATH"
-
-exec $python_exe -u -m mx "$@"
+exec $python_exe -u "$dir/mx.py" "$@"

mx.cmd

@@ -10,7 +10,5 @@ if defined MX_PYTHON (
   set "python_exe=python"
 )
 
-set "PYTHONPATH=%~dp0/src;%~dp0/oldnames"
-
 :: local variables can be used after endlocal if on the same line
-endlocal & "%python_exe%" -u -m mx %*
+endlocal & "%python_exe%" -u "%~dp0mx.py" %*

mx.py

@@ -0,0 +1,29 @@
+#!/usr/bin/env python
+"""
+Main entry point for running mx.
+"""
+
+import sys
+import runpy
+from pathlib import Path
+
+
+def patch_path():
+    """
+    Prepends the location of the main mx package as well as the `oldnames`
+    directory to `sys.path`.
+
+    We prepend because otherwise this file is recognized as the `mx` module and
+    not `src/mx`.
+    """
+    base_dir = Path(__file__).parent.absolute()
+    # Include the parent directory of this script in the python path variable,
+    # then allow loading of the mx module
+    sys.path.insert(0, str(base_dir / 'src'))
+    # The following allows the use of existing module names
+    sys.path.insert(0, str(base_dir / 'oldnames'))
+
+
+if __name__ == "__main__":
+    patch_path()
+    runpy.run_module("mx")

src/mx/_impl/mx.py

@@ -13781,10 +13781,9 @@ def run_mx(args, suite=None, mxpy=None, nonZeroIsFatal=True, out=None, err=None,
     :param suite: the primary suite or primary suite directory to use
     :param str mxpy: path the mx module to run (None to use the current mx module)
     """
-    if mxpy is not None:
-        # TODO GR-42789 Figure out how to deal with this argument
-        abort(f"mx.run_mx currently does not support custom mxpy {mxpy}")
-    commands = [get_mx_path(), '--java-home=' + get_jdk().home]
+    if mxpy is None:
+        mxpy = join(_mx_home, 'mx.py')
+    commands = [sys.executable, '-u', mxpy, '--java-home=' + get_jdk().home]
     cwd = None
     if suite:
         if isinstance(suite, str):