Auto merge of #23098 - TheGoddessInari:py2, r=jdm

Py2

<!-- Please describe your changes on the following line: -->
da3102338c: Rework mach.bat to support VS2019 and user-supplied environments.
4551f6031f: Default mach.bat to using py -2.
03e47081fe: Don't assume the user's environment in mach_bootstrap.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #23083 (GitHub issue number if applicable)

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it changes the python bootstrap slightly and the changes are obvious.

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

The virtualenv changes shouldn't disrupt anything as it just uses the existing python (being used) to call itself `-m virtualenv`, and only if it exists. imp is still apparently the preferred builtin way to find this in Python 2.x without actually importing it. Importing it would cause an unused import warning in tidy. It still picks up the new things in _virtualenv, just no longer has a special case for Win32/MSYS because it's no longer needed.

The .bat change is the simplest I could think of that allows fallback in both cases and is no worse than before. `where /Q` is documented by Microsoft to return 0 if a successful match is found, and print nothing in either case.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23098)
<!-- Reviewable:end -->

python/mach_bootstrap.py

@@ -78,14 +78,7 @@ CATEGORIES = {
 # Possible names of executables
 # NOTE: Windows Python doesn't provide versioned executables, so we must use
 # the plain names. On MSYS, we still use Windows Python.
-if sys.platform in ['msys', 'win32']:
-    PYTHON_NAMES = ["python"]
-    VIRTUALENV_NAMES = ["virtualenv"]
-    PIP_NAMES = ["pip"]
-else:
-    PYTHON_NAMES = ["python-2.7", "python2.7", "python2", "python"]
-    VIRTUALENV_NAMES = ["virtualenv-2.7", "virtualenv2.7", "virtualenv2", "virtualenv"]
-    PIP_NAMES = ["pip-2.7", "pip2.7", "pip2", "pip"]
+PYTHON_NAMES = ["python-2.7", "python2.7", "python2", "python"]
 
 
 def _get_exec_path(names, is_valid_path=lambda _path: True):
@@ -154,7 +147,7 @@ def wptserve_path(is_firefox, topdir, *paths):
 def _activate_virtualenv(topdir, is_firefox):
     virtualenv_path = os.path.join(topdir, "python", "_virtualenv")
     check_exec_path = lambda path: path.startswith(virtualenv_path)
-    python = _get_exec_path(PYTHON_NAMES)   # If there was no python, mach wouldn't have run at all!
+    python = sys.executable   # If there was no python, mach wouldn't have run at all!
     if not python:
         sys.exit('Failed to find python executable for starting virtualenv.')
 
@@ -162,11 +155,13 @@ def _activate_virtualenv(topdir, is_firefox):
     activate_path = os.path.join(virtualenv_path, script_dir, "activate_this.py")
     need_pip_upgrade = False
     if not (os.path.exists(virtualenv_path) and os.path.exists(activate_path)):
-        virtualenv = _get_exec_path(VIRTUALENV_NAMES)
-        if not virtualenv:
+        import imp
+        try:
+            imp.find_module('virtualenv')
+        except ImportError:
             sys.exit("Python virtualenv is not installed. Please install it prior to running mach.")
 
-        _process_exec([virtualenv, "-p", python, "--system-site-packages", virtualenv_path])
+        _process_exec([python, "-m", "virtualenv", "-p", python, "--system-site-packages", virtualenv_path])
 
         # We want to upgrade pip when virtualenv created for the first time
         need_pip_upgrade = True