Put the modules in a package and the scripts in a folder

Twenty-two files at the top of the tree was the first thing anybody saw of
this repository. They are one package now, imported relatively, and the three
scripts that are not the front door moved under scripts/. install.sh stays
where the README has always said it is.

What starts the application is dikte/__main__.py: python3 -m dikte runs it,
and so does naming the file, which is what the launcher symlink, both .desktop
files, the macOS bundle and every registered shortcut do. Run by path there is
no package around it, so it puts the checkout on sys.path itself.

The installers now keep the keys you chose when they are given none, which is
what an update is: update.sh no longer has to read them out and pass them back.
An updater from before this commit cannot read them at all, so the one thing it
can say, the default key with an empty discard key, is read as "nothing was
asked for" rather than obeyed. That guard can go once nobody is updating across
this commit.

tests/test_paste.py

@@ -22,7 +22,7 @@ import unittest
 from typing import ClassVar
 from unittest import mock
 
-import paste
+from dikte import paste
 from tests.support import DikteTest, FakeCompleted, only_these_tools