Fix locking in ReflectionComposablePart.ImportsCache

[omajid]

The second check needs to use the value from the field (to see updates made by other threads), not the local variable.

Fixes: #103650

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[omajid]

Would it be better to re-write ImportsCache and ImportValues to not use the local variable to make it closer to the easier-to-read representation suggested in https://github.com/dotnet/runtime/blob/main/docs/design/specs/Memory-model.md#examples-and-common-patterns under "Singleton (using a lock)"?

[stephentoub]

Thanks