[SwiftRefactor] PackageManifest: Add a public initializer to `FileSystem` #312

Job	Run time
Validate generated code	3m 8s
Run tests using swift-syntax-dev-utils (Linux)	24m 53s
Run tests using swift-syntax-dev-utils (Windows)	39m 15s
Test / Linux (6.1 - jammy)	4m 13s
Test / Linux (6.0 - jammy)	4m 3s
Test / Linux (5.10 - jammy)	4m 0s
Test / Linux (nightly-main - jammy)	4m 39s
Test / Linux (nightly-6.2 - jammy)	4m 51s
Test / Linux (5.9 - jammy)	3m 54s
Test / Embedded Swift SDK for Wasm Build (${{ matrix.swift_version }} - ${{ matrix.os_version }})	0s
Test / Static Linux Swift SDK Build (${{ matrix.swift_version }} - ${{ matrix.os_version }})	0s
Test / Windows (5.9 - windows-2022)	14m 12s
Test / Windows (nightly - windows-2022)	20m 49s
Test / Windows (nightly-6.2 - windows-2022)	16m 24s
Test / Windows (6.1 - windows-2022)	13m 1s
Test / Windows (6.0 - windows-2022)	12m 42s
Soundness / License headers check	12s
Soundness / YAML lint check	5s
Soundness / Shell check	34s
Soundness / Format check	43s
Soundness / Broken symlinks check	6s
Soundness / Unacceptable language check	4s
Soundness / Python lint check	5s
Soundness / API breakage check	-1s
Test / Swift SDK for Wasm Build (${{ matrix.swift_version }} - ${{ matrix.os_version }})	-1s
Test / macOS (Xcode ${{ matrix.xcode_version }} - ${{ matrix.os_version }} - ${{ matrix.arch }})	-1s
Soundness / Documentation check	4m 42s
	2h 56m 32s

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