Move SectorAllocator into pkg/filesystem/pool

[EdSchouten]

SectorAllocator is only used by the FilePool code, so there's no point in keeping it in pkg/filesystem.

[aspect-workflows]

Test

16 test targets passed

Targets

//pkg/blobstore:blobstore_test [k8-fastbuild]                         173ms
//pkg/builder:builder_test [k8-fastbuild]                             84ms
//pkg/cas:cas_test [k8-fastbuild]                                     111ms
//pkg/cleaner:cleaner_test [k8-fastbuild]                             191ms
//pkg/clock:clock_test [k8-fastbuild]                                 68ms
//pkg/filesystem/pool:pool_test [k8-fastbuild]                        66ms
//pkg/filesystem/virtual/fuse:fuse_test [k8-fastbuild]                130ms
//pkg/filesystem/virtual/nfsv4:nfsv4_test [k8-fastbuild]              138ms
//pkg/filesystem/virtual:virtual_test [k8-fastbuild]                  171ms
//pkg/filesystem:filesystem_test [k8-fastbuild]                       76ms
//pkg/runner:runner_test [k8-fastbuild]                               256ms
//pkg/scheduler/initialsizeclass:initialsizeclass_test [k8-fastbuild] 93ms
//pkg/scheduler/platform:platform_test [k8-fastbuild]                 82ms
//pkg/scheduler/routing:routing_test [k8-fastbuild]                   95ms
//pkg/scheduler:scheduler_test [k8-fastbuild]                         185ms
//pkg/sync:sync_test [k8-fastbuild]                                   68ms

Total test execution time was 2s. 3 tests (15.8%) were fully cached saving 305ms.