Bug 1842918 - normalize away spaces in `$(PYTHON3)` r=firefox-build-system-reviewers,glandium

As of bug 1838763, Python virtualenvs are now located in a subdirectory
of `$MOZBUILD_STATE_PATH`. This, in turn, defaults to a subdirectory of
the home directory, which on Windows often contains spaces.

The Windows build instructions were updated to add a requirement that
`MOZBUILD_STATE_PATH` not contain spaces, under the assumption that this
was merely a clarification of an existing restriction; but this is
indeed a new requirement and breaks existing setups.

Instead, allow makefiles to safely invoke `$(PYTHON3)` without quoting
it everywhere (as they do) by ensuring that its value contains no
spaces, by using the DOS-style (8.3) short path to the Python
executable.

Additionally, clarify in the documentation exactly which paths are
required not to contain spaces.

Differential Revision: https://phabricator.services.mozilla.com/D183305

build/moz.configure/init.configure

@@ -172,7 +172,7 @@ def virtualenv_python3():
     return namespace(
         # sys.executable is currently not updated for in-process activations. However,
         # sys.prefix is, so we can calculate the python executable's path from there.
-        path=normsep(PythonVirtualenv(realpath(sys.prefix)).python_path),
+        path=normalize_path(PythonVirtualenv(realpath(sys.prefix)).python_path),
         str_version=".".join(str(i) for i in sys.version_info[0:3]),
     )
 

docs/setup/windows_build.rst

@@ -151,19 +151,11 @@ they may be resolved by `upgrading your MozillaBuild <https://wiki.mozilla.org/M
 Spaces in folder names
 ~~~~~~~~~~~~~~~~~~~~~~
 
-**Firefox will not build** if the path to the installation
-tool folders contains **spaces** or other breaking characters such as
-pluses, quotation marks, or metacharacters.  The Visual Studio tools and
-SDKs are an exception - they may be installed in a directory which
-contains spaces. It is strongly recommended that you accept the default
-settings for all installation locations.
-
-If your Windows username contains a space, the default ``.mozbuild`` path
-will also contain a space. You will need to move where ``.mozbuild`` directory
-lives by setting the ``MOZBUILD_STATE_PATH`` environment variable to a location
-without a space in the path (eg: ``MOZBUILD_STATE_PATH=C:\.mozbuild``). If your
-``.mozbuild`` already contains bootstrapped dependencies, you should copy or move it
-to that new location, otherwise you will need to run ``./mach bootstrap`` again.
+**Firefox will not build** if the path to MozillaBuild or the Firefox source
+contain **spaces** or other breaking characters such as pluses, quotation marks,
+or metacharacters.  The Visual Studio tools and SDKs are an exception - they may
+be installed in a directory which contains spaces. It is strongly recommended
+that you accept the default settings for all installation locations.
 
 Quotation marks in ``PATH``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~