Remove the option to create a standalone Executor

[johny-b]

NOTE: Recommended way of reading this PR is one commit at a time. Commits are quite separate and it's possible that one of them will be totally wrong while others will make sense.

Major things done:

• Standalone Executor was replaced with a Golem instance in test_payment_platform tests
• Standalone Executor was replaced with a Golem instance in a bunch of goth tests
• Executor now has only one way of instantiation, that requires passing _engine

Additional cleanup:

• Removed budget from yapapi.Golem.execute_tasks. This is a backward-incompatible change, but this param was ignored either way (was passed to the Executor along the _engine, but Executor ignored budget when _engine was provided). So I think this is fine.
• Removed depreciated package param from Executor.__init__
• Executor is no longer an AsyncContextManager. I changed this because there was nothing in __aenter__/__aexit__ that couldn't be removed/replaced without any additional cost.

[shadeofblue]

* Removed depreciated `package` param from `Executor.__init__`

quick nitpick: it's "deprecated", not "depreciated" ...

[shadeofblue]