refactor: Extract store package

[jdkaplan]

Now that we've cleaned up database.go a bit, it looks like a good candidate for extraction into its own package. This makes it easier to test and explain to new contributors.

I named the package store because firestore would have collided with the actual Firestore client package and fs would have collided with the actual standard library filesystem package. We can re-refactor into a better name if we find one!

Some extra changes I made along the way:

• Rename FirestoreFooDB to store.FooClient
• Scope SecretsClient to its one collection
• Extract a pbtest package now that the test Firestore client is used in two test packages.
• Use our new assert.Equal helper instead of defining .Equal methods on DB structs.

[cceckman]

Nice!

I have some style thoughts for a future cleanup:

• Early-bind the firestoreClient, so we don't have to deal with pl.db in every call
• Some places with arguments recurser.ID, recurser -- seems like a redundant argument?

Maybe I'll just send the PR :)

[cceckman]

* Early-bind the `firestoreClient`, so we don't have to deal with `pl.db` in every call

Er, I guess this is what we're getting away from: having four different objects in the top-level. Hm.

[jdkaplan]

• Early-bind the firestoreClient, so we don't have to deal with pl.db in every call

Er, I guess this is what we're getting away from: having four different objects in the top-level. Hm.

Yeah, I go back-and-forth on this one a lot.

My motivation for doing it in this PR was to make the "constructor" for PairingLogic easier, because I'm working toward a dedicated pairing package to host it.

I don't mind the struct itself having all four of them early-bound, though! I just don't like main knowing that. Once it has a constructor, I wouldn't mind if we undo the late-binding change.

• Some places with arguments recurser.ID, recurser -- seems like a redundant argument?

Agreed! There's even one place I know that the first arg is (now) even totally unused:

pairing-bot/store/recursers.go

Line 114 in 235495c

func (r *RecursersClient) Set(ctx context.Context, _ int64, recurser *Recurser) error {

Maybe I'll just send the PR :)

Yes, please! Or if not, we can offer it as a "good first issue" for someone who wants to get started learning Go. Either way makes me happy! :)