packaging: isolate invocation of WiX to own function

Like we did for Inno, we want to split out the building
of Mercurial from invoking the packaging tool so that we
can introduce an alternate build mechanism.

As part of this refactor, there are inconsequential changes
to file layouts. Before, some shared files such as the
WiX binaries and merge modules would be installed under
build/. Now, they are installed under build/wix-*. This
is to keep implementation simpler. But it also helps keep
build state more isolated.

Differential Revision: https://phab.mercurial-scm.org/D8474

contrib/packaging/hgpackaging/wix.py

@@ -341,7 +341,6 b' def build_installer('
 
     hg_build_dir = source_dir / 'build'
     dist_dir = source_dir / 'dist'
-    wix_dir = source_dir / 'contrib' / 'packaging' / 'wix'
 
     requirements_txt = (
         source_dir / 'contrib' / 'packaging' / 'requirements_win32.txt'
@@ -388,13 +387,41 b' def build_installer('
             print('removing %s' % p)
             p.unlink()
 
-    wix_pkg, wix_entry = download_entry('wix', hg_build_dir)
-    wix_path = hg_build_dir / ('wix-%s' % wix_entry['version'])
+    return run_wix_packaging(
+        source_dir,
+        build_dir,
+        staging_dir,
+        arch,
+        version=version,
+        orig_version=orig_version,
+        msi_name=msi_name,
+        extra_wxs=extra_wxs,
+        extra_features=extra_features,
+    )
+
+
+def run_wix_packaging(
+    source_dir: pathlib.Path,
+    build_dir: pathlib.Path,
+    staging_dir: pathlib.Path,
+    arch: str,
+    version: str,
+    orig_version: str,
+    msi_name: typing.Optional[str] = "mercurial",
+    extra_wxs: typing.Optional[typing.Dict[str, str]] = None,
+    extra_features: typing.Optional[typing.List[str]] = None,
+):
+    """Invokes WiX to package up a built Mercurial."""
+
+    wix_dir = source_dir / 'contrib' / 'packaging' / 'wix'
+
+    wix_pkg, wix_entry = download_entry('wix', build_dir)
+    wix_path = build_dir / ('wix-%s' % wix_entry['version'])
 
     if not wix_path.exists():
         extract_zip_to_directory(wix_pkg, wix_path)
 
-    ensure_vc90_merge_modules(hg_build_dir)
+    ensure_vc90_merge_modules(build_dir)
 
     source_build_rel = pathlib.Path(os.path.relpath(source_dir, build_dir))
 
@@ -413,7 +440,7 b' def build_installer('
     source = wix_dir / 'mercurial.wxs'
     defines['Version'] = version
     defines['Comments'] = 'Installs Mercurial version %s' % version
-    defines['VCRedistSrcDir'] = str(hg_build_dir)
+    defines['VCRedistSrcDir'] = str(build_dir)
     if extra_features:
         assert all(';' not in f for f in extra_features)
         defines['MercurialExtraFeatures'] = ';'.join(extra_features)