Add initial hook-based check execution mechanism

[xmunoz]

Progress on #7093

[ewdurbin]

One option rather than instantiating checks directly from the upload method would be to add a db listener:

diff --git a/warehouse/malware/__init__.py b/warehouse/malware/__init__.py
index 0d989ac0..b4b4973b 100644
--- a/warehouse/malware/__init__.py
+++ b/warehouse/malware/__init__.py
@@ -10,8 +10,32 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from warehouse import db
 from warehouse.malware.interfaces import MalwareCheckService
 from warehouse.malware.services import database_malware_check_factory
+from warehouse.packaging.models import File, Project, Release
+
+
+@db.listens_for(db.Session, "after_flush")
+def determine_malware_checks(config, session, flush_context):
+
+    malware_checks = session.info.setdefault("warehouse.malware.checks", set())
+
+    for obj in session.new:
+        if obj.__class__ in (File,):
+            malware_checks.update([f"example_file_check:{obj.id}"])
+        if obj.__class__ in (Release,):
+            malware_checks.update([f"example_release_check:{obj.id}"])
+        if obj.__class__ in (Project,):
+            malware_checks.update([f"example_project_check:{obj.id}"])
+
+
+@db.listens_for(db.Session, "after_commit")
+def queue_malware_checks(config, session):
+
+    malware_checks = session.info.setdefault("warehouse.malware.checks", set())
+
+    print(malware_checks)
+    # enqueue tasks
 
 
 def includeme(config):

This is similar to how our cache purging works. The example above is a bit rough, but implementing it in this way would allow for instantiating the checks from the event and even instantiating different checks based on what changed.

[ewdurbin]

I'm not sure that any of the above concerns are blockers to merge here.