packaging: add support for PyOxidizer

I've successfully built Mercurial on the development tip of
PyOxidizer on Linux and Windows. It mostly "just works" on Linux.
Windows is a bit more finicky.

In-memory resource files are probably not all working correctly
due to bugs in PyOxidizer's naming of modules. PyOxidizer now
now supports installing files next to the produced binary. (We
do this for templates in the added file.) So a workaround
should be available.

Also, since the last time I submitted support for PyOxidizer,
PyOxidizer gained the ability to auto-generate Rust projects
to build executables. So we don't need to worry about vendoring
any Rust code to initially support PyOxidizer. However, at some
point we will likely want to write our own command line driver
that embeds a Python interpreter via PyOxidizer so we can run
Rust code outside the confines of a Python interpreter. But that
will be a follow-up.

I would also like to add packaging.py CLI commands to build
PyOxidizer distributions. This can come later, if ever.
PyOxidizer's new "targets" feature makes it really easy to define
packaging tasks in its Starlark configuration file. While not
much is implemented yet, eventually we should be able to produce
MSIs, etc using a `pyoxidizer build` one-liner. We'll get there...

Differential Revision: https://phab.mercurial-scm.org/D7450

contrib/packaging/pyoxidizer.bzl

@@ -0,0 +1,57 b''
+# Instructions:
+#
+# 1. cargo install --version 0.5.0 pyoxidizer
+# 2. cd /path/to/hg
+# 3. pyoxidizer build --path contrib/packaging [--release]
+# 4. Run build/pyoxidizer/<arch>/<debug|release>/app/hg
+#
+# If you need to build again, you need to remove the build/lib.* and
+# build/temp.* directories, otherwise PyOxidizer fails to pick up C
+# extensions. This is a bug in PyOxidizer.
+
+ROOT = CWD + "/../.."
+
+set_build_path(ROOT + "/build/pyoxidizer")
+
+def make_exe():
+    dist = default_python_distribution()
+
+    code = "import hgdemandimport; hgdemandimport.enable(); from mercurial import dispatch; dispatch.run()"
+
+    config = PythonInterpreterConfig(
+        raw_allocator = "system",
+        run_eval = code,
+        # We need this to make resourceutil happy, since it looks for sys.frozen.
+        sys_frozen = True,
+    )
+
+    exe = dist.to_python_executable(
+        name = "hg",
+        config = config,
+    )
+
+    # Use setup.py install to build Mercurial and collect Python resources to
+    # embed in the executable.
+    resources = dist.setup_py_install(ROOT)
+    exe.add_python_resources(resources)
+
+    return exe
+
+def make_install(exe):
+    m = FileManifest()
+
+    # `hg` goes in root directory.
+    m.add_python_resource(".", exe)
+
+    templates = glob(
+        include=[ROOT + "/mercurial/templates/**/*"],
+        strip_prefix = ROOT + "/mercurial/",
+    )
+    m.add_manifest(templates)
+
+    return m
+
+register_target("exe", make_exe)
+register_target("app", make_install, depends = ["exe"], default = True)
+
+resolve_targets()