Add option for pre_conda shell script

This is needed for E3SM-Unified activation scripts on ALCF Polaris
because its `xalt` module interferes with even the most basic
`conda` operations such as `conda activate`.

Author: xylar

mache/machines/pre_conda/__init__.py

@@ -0,0 +1,23 @@
+from importlib.resources import read_text
+
+
+def load_pre_conda_script(machine, ext):
+    """
+    Return the contents of a shell script to prepend before loading conda
+
+    Parameters
+    ----------
+    machine : str
+        The name of the machine to load the script for.
+    ext : str
+        The file extension of the script, either 'sh' or 'csh'.
+
+    Returns
+    -------
+    str
+        The contents of the shell script.
+    """
+    try:
+        return read_text('mache.machines.pre_conda', f'{machine}.{ext}')
+    except FileNotFoundError:
+        return ''

mache/machines/pre_conda/polaris.csh

@@ -0,0 +1 @@
+module unload xalt

mache/machines/pre_conda/polaris.sh

@@ -0,0 +1 @@
+module unload xalt