Restructure storage filesystem directory

[rajkan01]

Summary of changes

Preceding PR: #13253 #13244 #13273

Restructured storage/filesystem as per the new directory structure proposal:

storage
├── blockdevice
├── filesystem
│   ├── mbed_lib.json
│   ├── include
│   │   └── filesystem
│   │       ├── mbed_filesystem.h
│   │       ├── Dir.h
│   │       └── [...]
│   ├── source
│   │   ├── Dir.cpp
│   │   └── [...]
│   ├── tests
│   ├── fat
│   │   ├── include
│   │   │   └── fat
│   │   ├── source
│   │   ├── tests
│   │   └── [...]
│   ├── littlefs
│   │   └── [...]
│   └── littlefsv2
│       └── [...]

Impact of changes

None.

Migration actions required

None.

Documentation

To Be Updated.

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[x] Tests / results supplied as part of this PR

Manual testing: (Build for K64F target with GCC_ARM toolchain)

• mbed-os-example-blockdevice
• mbed-os-example-filesystem
• Greentea tests: Full and Bare metal profiles
• Unit tests build and run

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Reviewers

@bulislaw @0xc0170 @ARMmbed/mbed-os-core

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[ciarmcom]

@rajkan01, thank you for your changes.
@ARMmbed/mbed-os-storage @ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.