Refactor concurrency so that it is pluggable

However, this may not be the best idea because it involves global
state that could be overridden by a parallel call to ocrmypdf.ocr.

src/ocrmypdf/pluginspec.py

@@ -9,7 +9,7 @@ from abc import ABC, abstractmethod, abstractstaticmethod
 from argparse import ArgumentParser, Namespace
 from collections import namedtuple
 from pathlib import Path
-from typing import TYPE_CHECKING, AbstractSet, List, Optional
+from typing import TYPE_CHECKING, AbstractSet, Callable, Iterable, List, Optional
 
 import pluggy
 
@@ -62,6 +62,47 @@ def check_options(options: Namespace) -> None:
     """
 
 
+class ParallelExecutor(ABC):
+    @abstractstaticmethod
+    def __call__(
+        use_threads: bool,
+        max_workers: int,
+        tqdm_kwargs: dict,
+        worker_initializer: Callable,
+        task: Callable,
+        task_finished: Callable,
+        task_arguments: Optional[Iterable] = None,
+    ):
+        """
+        Args:
+            use_threads: If False, the workload is the sort that will benefit from
+                running in a multiprocessing context (for example, it uses Python
+                heavily, and parallelizing it with threads is not expected to be
+                performant).
+            max_workers: The maximum number of workers that should be run.
+            tdqm_kwargs: Arguments to set up the progress bar.
+            worker_initializer: Called when the worker is initialized, in the worker's
+                execution context. Must be possible to marshall to the worker.
+            task: Called when the worker starts a new task, in the worker's execution
+                context. Must be possible to marshallable to the worker.
+            task_finished: Called when a worker finishes a task, in the parent's
+                context.
+            task_arguments: An iterable that generates a group of parameters for each
+                task. This runs in the parent's context, but the parameters must be
+                marshallable to the worker.
+        """
+
+
+@hookspec(firstresult=True)
+def get_parallel_executor() -> Callable:
+    """Called to perform parallel execution
+
+    This may be used to replace OCRmyPDF's default parallel execution system
+    with a third party alternative. For example, you could make OCRmyPDF run in a
+    distributed environment.
+    """
+
+
 @hookspec
 def validate(pdfinfo: 'PdfInfo', options: Namespace) -> None:
     """Called to give a plugin an opportunity to review *options* and *pdfinfo*.