ExecuteSQLOp rendering respects alembic_module_prefix

ExecuteSQLOp rendering respects alembic_module_prefix

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### Description

Rendering ExecuteSQLOp did not respect the `alembic_module_prefix` setting due
to having `op.` hard-coded in its template.

This change updates the render logic to use the same
`_alembic_autogenerate_prefix()` as it seems the rest of the ops use.

### Checklist

This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [x] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

Fixes #1656

Closes: #1657
Pull-request: https://github.com/sqlalchemy/alembic/pull/1657
Pull-request-sha: c67f4b7ce25c363d525cd2483370b0be024160d9

Change-Id: I6ca36642ed3755a2d96824498502ada6ae16f714