packaging: split Inno installer building from Mercurial building

We want to make the logic for producing the installer agnostic about
how Mercurial is built to allow for alternate build methods (like
PyOxidizer).

Differential Revision: https://phab.mercurial-scm.org/D8472

contrib/packaging/hgpackaging/inno.py

@@ -61,8 +61,7 b' def build('
 
     vc_x64 = r'\x64' in os.environ.get('LIB', '')
     arch = 'x64' if vc_x64 else 'x86'
-    inno_source_dir = source_dir / 'contrib' / 'packaging' / 'inno'
-    inno_build_dir = build_dir / ('inno-%s' % arch)
+    inno_build_dir = build_dir / ('inno-py2exe-%s' % arch)
     staging_dir = inno_build_dir / 'stage'
 
     requirements_txt = (
@@ -104,6 +103,31 b' def build('
         print('copying %s to %s' % (f, dest_path))
         shutil.copyfile(f, dest_path)
 
+    build_installer(
+        source_dir,
+        inno_build_dir,
+        staging_dir,
+        iscc_exe,
+        version,
+        arch="x64" if vc_x64 else None,
+    )
+
+
+def build_installer(
+    source_dir: pathlib.Path,
+    inno_build_dir: pathlib.Path,
+    staging_dir: pathlib.Path,
+    iscc_exe: pathlib.Path,
+    version,
+    arch=None,
+):
+    """Build an Inno installer from staged Mercurial files.
+
+    This function is agnostic about how to build Mercurial. It just
+    cares that Mercurial files are in ``staging_dir``.
+    """
+    inno_source_dir = source_dir / "contrib" / "packaging" / "inno"
+
     # The final package layout is simply a mirror of the staging directory.
     package_files = []
     for root, dirs, files in os.walk(staging_dir):
@@ -158,8 +182,8 b' def build('
 
     args = [str(iscc_exe)]
 
-    if vc_x64:
-        args.append('/dARCH=x64')
+    if arch:
+        args.append('/dARCH=%s' % arch)
 
     if not version:
         version = read_version_py(source_dir)