pyinstaller: Use qutebrowser/__main__.py

When we use qutebrowser.py, PyInstaller 3.2 gets confused
somehow (because of the name conflict with qutebrowser/ maybe?) and
doesn't do anything when running the bundled app.

With qutebrowser/__main__.py the generated filename is still correct and
it actually works.

misc/qutebrowser.spec

@@ -37,7 +37,7 @@ else:
     icon = None
 
 
-a = Analysis(['../qutebrowser.py'],
+a = Analysis(['../qutebrowser/__main__.py'],
              pathex=['misc'],
              binaries=None,
              datas=get_data_files(),