Equivalent to #2082.
This change will help continue debugging [CSDK-517].

While debugging locally I ran into this crash:
```
Test Case '-[StoreKitUnitTests.UserDefaultsDefaultTests testDefaultIsRevenueCatSuiteIfStandardDoesNotContainUserID]' started.
RevenueCat/DeviceCache.swift:85: Fatal error: [Purchases] - Cached appUserID has been deleted from user defaults.
This leaves the SDK in an undetermined state. Please make sure that RevenueCat
entries in user defaults don't get deleted by anything other than the SDK.
More info: https://rev.cat/userdefaults-crash
```

But that should have been impossible because the current test had not initialized `Purchases` or `DeviceCache`, which showed that there was a leaking `DeviceCache` from another test.

…tances in memory

We allow calling `Purchases.configure` multiple times. Likely a programmer error, but it's common in tests, or for example in `Purchase Tester` where users can re-configure the SDK at runtime (#1999).
When this happens, the SDK was creating the new instance before clearing the old one from memory (clear thanks to #2082, notice how the first `deinit` happens after the second `init`):
```
[Purchases] - VERBOSE: Purchases.init: created new Purchases instance: Purchases (0x0000600003ca4200)
[Purchases] - INFO: ℹ️ Purchases instance already set. Did you mean to configure two Purchases objects?
[Purchases] - VERBOSE: Purchases.init: created new Purchases instance: Purchases (0x0000600003c8a100)
[Purchases] - VERBOSE: Purchases.deinit: Purchases (0x0000600003ca4200)
[Purchases] - VERBOSE: Purchases.deinit: Purchases (0x0000600003c8a100)
```

With this change, we avoid that, which could also avoid issues with concurrent requests ([CSDK-517]):
```
[Purchases] - VERBOSE: Purchases.init: created new Purchases instance: Purchases (0x0000600002b6e600)
[Purchases] - INFO: ℹ️ Purchases instance already set. Did you mean to configure two Purchases objects?
[Purchases] - VERBOSE: Purchases.deinit: Purchases (0x0000600002b6e600)
[Purchases] - VERBOSE: Purchases.init: created new Purchases instance: Purchases (0x0000600002b68100)
[Purchases] - VERBOSE: Purchases.deinit: Purchases (0x0000600002b68100)
```

…tances in memory (#2180)

We allow calling `Purchases.configure` multiple times. Likely a
programmer error, but it's common in tests, or for example in `Purchase
Tester` where users can re-configure the SDK at runtime (#1999), which
is where I saw this issue.

When this happens, the SDK was creating the new instance before clearing
the old one from memory (clear thanks to #2082). Notice how the first
`deinit` happens **after** the second `init`:
```
[Purchases] - VERBOSE: Purchases.init: created new Purchases instance: Purchases (0x0000600003ca4200)
[Purchases] - INFO: ℹ️ Purchases instance already set. Did you mean to configure two Purchases objects?
[Purchases] - VERBOSE: Purchases.init: created new Purchases instance: Purchases (0x0000600003c8a100)
[Purchases] - VERBOSE: Purchases.deinit: Purchases (0x0000600003ca4200)
[Purchases] - VERBOSE: Purchases.deinit: Purchases (0x0000600003c8a100)
```

With this change, we avoid that, which could also avoid issues with
concurrent requests ([CSDK-517]):
```
[Purchases] - VERBOSE: Purchases.init: created new Purchases instance: Purchases (0x0000600002b6e600)
[Purchases] - INFO: ℹ️ Purchases instance already set. Did you mean to configure two Purchases objects?
[Purchases] - VERBOSE: Purchases.deinit: Purchases (0x0000600002b6e600)
[Purchases] - VERBOSE: Purchases.init: created new Purchases instance: Purchases (0x0000600002b68100)
[Purchases] - VERBOSE: Purchases.deinit: Purchases (0x0000600002b68100)
```

Unfortunately we don't have an easy way to test this, especially
considering that we prohibit multiple instances in tests (#2100).

[CSDK-517]:
https://revenuecats.atlassian.net/browse/CSDK-517?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ