docs: import Command and Application from where they live

[melbinjp]

src/cleo/__init__.py defines only __version__ and exports nothing, so every documented example fails on its first line:

>>> from cleo import Command
ImportError: cannot import name 'Command' from 'cleo'

That includes the first code block in the introduction, the one a new user copies before anything else works.

Six occurrences, three documents

file	line	was
docs/introduction.rst	9	from cleo import Command
docs/introduction.rst	43, 409, 433	from cleo import Application
docs/single_command_tool.rst	10	from cleo import Application
docs/usage.rst	17	from cleo import Application

Corrected to the paths the project's own tests use

from cleo.commands.command import Command
from cleo.application import Application

Both were executed against this tree before pushing: the old import raises, the new ones resolve.

Found with docproof, which checks documentation claims against the code. It flagged cleo.Command as not defined, imported, or re-exported anywhere in cleo; the other five turned up when I went looking for the same shape.