The event loop (even after it was brought into one place in _run in the
previous commit) was hard to follow. The states and transitions weren't
clear or documented, and the privilege escalation code was non-blocking
while the rest was blocking.

Now we have a state machine with four states: awaiting_prompt,
awaiting_escalation, ready_to_send (initial data), and awaiting_exit.
The actions in each state and the transitions between then are clearly
documented.

The check_incorrect_password() method no longer checks for empty strings
(since they will always match), and check_become_success() uses equality
rather than a substring match to avoid thinking an echoed command is an
indication of successful escalation. Also adds a check_missing_password
connection method to detect the error from sudo -n/doas -n.

The main exec_command/put_file/fetch_file methods now _build_command and
call _run to handle input from/output to the ssh process. The purpose is
to bring connection handling together in one place so that the locking
doesn't have to be split across functions.

Note that this doesn't change the privilege escalation and connection IO
code at all—just puts it all into one function.

Most of the changes are just moving code from one place to another (e.g.
from _connect to _build_command, from _exec_command and _communicate to
_run), but there are some other notable changes:

1. We test for the existence of sshpass the first time we need to use
   password authentication, and remember the result.
2. We set _persistent in _build_command if we're using ControlPersist,
   for later use in close(). (The detection could be smarter.)
3. Some apparently inadvertent inconsistencies between put_file and
   fetch_file (e.g. argument quoting, sftp -b use) have been removed.

Also reorders functions into a logical sequence, removes unused imports
and functions, etc.

Aside: the high-level EXEC/PUT/FETCH description should really be logged
from ConnectionBase, while individual subclasses log transport-specific
details.

Fixes #12484

* Make LookupBase an abc with required methods (run()) marked as an
  abstractmethod
* Mark methods that don't use self as @staticmethod
* Document how to implement the run method of a lookup plugin.

Add mention of roles_path parameter to `ansible-galaxy install` command

Fixes #12464

Fix for user defined modules not overriding modules from core.

This fix takes into account that powershell modules are somewhat
different than regular modules and have to be kept separate.

Fixes #12465

Python 3: there's no basestring

Fix option descriptions in ansible-doc output

Fixes #12462.

Fixes #12460

Fixes one failing test.

Now technically a filename can be a bytestring, even on Python 3.  I
hope this is unlikely for Ansible.

Without rebreaking #12055

Fixes #ansible-modules-core/2098

Detect if core modules aren't installed and warn if that is the  case

Fixes #11206

Fix typo

Follow up to 8769f03c, which allows the undefined var error to be raised
if we're getting vars with a full context (play/host/task) and the host
has already gathered facts. In this way, vars_files containing variables
that fail to be templated are not silently ignored.

Python 3: there is no 'basestring'

Fixes #12449

Close module file once we're done reading from it

This fixes a failing unit test.

In actual use (which is still quite far), I'm not sure if bytes ->
unicode conversion should be done here (in which case the code will fail
with an AttributeError: 'bytes' object has no attribute 'readlines'), or
inside self._connection.exec_command() (in which case my change is
correct).

Use dict.get()

Stops using _groups_list to check for host when using 'add_host'