Use importlib.metadata to read requirements (#12088)

* Pull runtime dep checks into their own module
* Reimplement `check_requirements` using `importlib`

I've tried to make this clearer. We start by working out which of
Synapse's requirements we need to be installed here and now. I was
surprised that there wasn't an easier way to see which packages were
installed by a given extra.

I've pulled out the error messages into functions that deal with "is
this for an extra or not". And I've rearranged the loop over two
different sets of requirements into one loop with a "must be instaled"
flag.

I hope you agree that this is clearer.

* Test cases

1 files changed, 3 insertions, 3 deletions

diff --git a/synapse/app/__init__.py b/synapse/app/__init__.py
index ee51480a9e..334c3d2c17 100644
--- a/synapse/app/__init__.py
+++ b/synapse/app/__init__.py
@@ -15,13 +15,13 @@ import logging
 import sys
 from typing import Container
 
-from synapse import python_dependencies  # noqa: E402
+from synapse.util import check_dependencies
 
 logger = logging.getLogger(__name__)
 
 try:
-    python_dependencies.check_requirements()
-except python_dependencies.DependencyException as e:
+    check_dependencies.check_requirements()
+except check_dependencies.DependencyException as e:
     sys.stderr.writelines(
         e.message  # noqa: B306, DependencyException.message is a property
     )