Let other modules modify the scanning rules

Problem/Motivation

One of the changes in #3214186: Scan for snippets of process plugins is

--- a/src/Plugin/MigrationPluginManager.php
+++ b/src/Plugin/MigrationPluginManager.php
@@ -23,7 +23,7 @@ class MigrationPluginManager extends DefaultMigrationPluginManager {
       }, $this->moduleHandler->getModuleDirectories());
 
       // Retrieves **/*.yml files but gets rid of /migrations/state/*.yml files.
-      $yaml_discovery = new YamlRecursiveDirectoryDiscovery($directories, 'migrate', 'id', '#/migrations/state/#');
+      $yaml_discovery = new YamlRecursiveDirectoryDiscovery($directories, 'migrate', 'id', '#/migrations/(process|state)/#');

That does not scale well. I think we need an easy way for a developer to set exclude rules while working on a custom migration.

It might be useful to allow include rules as well as exclude rules. For example, a developer working on several related sites might want a single module (perhaps including some custom plugins) and migrations in migrations/site1/, migrations/site2/, ...

Proposed resolution

Either add an alter hook or trigger an event so that other modules can modify the paths to be scanned.

I think we should optimize for simplicity, not flexibility. Instead of giving other modules the list of directories and letting them manipulate it, just let them add include/exclude patterns.

Remaining tasks

User interface changes

API changes

This feature will add to the public API of this module.

Data model changes