Flatten the bridge/opentracing/internal package into bridge/opentracing

[MrAlias]

This package only contains mocks for testing. These are only ever used in the opentracing bridge.

• Copy bridge/opentracing/internal/mock.go to bridge/opentracing/mock_test.go
• Rename all exported types in bridge/opentracing/mock.go to be unexported:
  • ComponentKey -> componentKey
  • ServiceKey -> serviceKey
  • StatusCodeKey -> statusCodeKey
  • StatusMessageKey -> statusMessageKey
  • ErrorKey -> errorKey
  • NameKey -> nameKey
  • MockContextKeyValue -> mockContextKeyValue
  • MockTracer -> mockTracer
  • NewMockTracer -> newMockTracer
  • MockEvent -> mockEvent
  • MockLink -> mockLink
  • MockSpan -> mockSpan
• Update all references in bridge/opentracing to use the locally defined code
• Remove bridge/opentracing/internal

[MrAlias]

Assigned to @Leoff00