Update for provider refactor
Update for provider refactor

Need to import perfkitbenchmarker.providers.aws rather than
perfkitbenchmarker.aws, and similarly for the other providers.

GoogleCloudPlatform/PerfKitBenchmarker,mateusz-blaszkowski/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,mateusz-blaszkowski/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker