Revert "Revert "Deprecate FileSystemProvider and RenderNodeProvider""

[d-ronnqvist]

Reverts #1116

---

First I locally verified that the unmodified code in 4250efe fails with this SIL Validation compiler error in release builds using swift-DEVELOPMENT-SNAPSHOT-2024-11-09-a which is tracked by swiftlang/swift#77955:

SIL verification failed: Should not have an operand for the opened existential: AMI->getTypeDependentOperands().empty()
Verifying instruction:
     %5 = open_existential_addr immutable_access %4 : $*any FileManagerProtocol to $*@opened("0C866184-B2ED-11EF-9D62-7287C2F98703", any FileManagerProtocol) Self // users: %19, %15, %15, %14
->   %19 = witness_method $@_opaqueReturnTypeOf("$s9SwiftDocC19FileManagerProtocolPAAE14recursiveFiles13startingPoint7optionsQr20FoundationEssentials3URLV_AG0cD0C27DirectoryEnumerationOptionsVtF", 0) __<@opened("0C866184-B2ED-11EF-9D62-7287C2F98703", any FileManagerProtocol) Self>, #Sequence.makeIterator : <Self where Self : Sequence> (__owned Self) -> () -> Self.Iterator, %5 : $*@opened("0C866184-B2ED-11EF-9D62-7287C2F98703", any FileManagerProtocol) Self : $@convention(witness_method: Sequence) <τ_0_0 where τ_0_0 : Sequence> (@in τ_0_0) -> @out τ_0_0.Iterator // type-defs: %5; user: %21
     %21 = apply %19<@opened("0C86668E-B2ED-11EF-9D62-7287C2F98703", any Sequence<URL>) Self>(%20, %17) : $@convention(witness_method: Sequence) <τ_0_0 where τ_0_0 : Sequence> (@in τ_0_0) -> @out τ_0_0.Iterator // type-defs: %16
In function:
// StaticHostableTransformer.transform()
// Isolation: unspecified
sil @$s18SwiftDocCUtilities25StaticHostableTransformerV9transformyyKF : $@convention(method) (@in_guaranteed StaticHostableTransformer) -> @error any Error {
[%0: read v**, write s3.x.v**, copy v**, destroy s3.x.v**]
[global: read,write,copy,destroy,allocate,deinit_barrier]
...

Then I modified the declaration of (extension in SwiftDocC):SwiftDocC.FileManagerProtocol.recursiveFiles(startingPoint: FoundationEssentials.URL, options: FoundationEssentials.FileManager.DirectoryEnumerationOptions) -> some in fbda331 to remove the existential type (using an explicit IteratorSequence<_FilesIterator> return value instead) and verified that the same compiler no longer fails with the same release build invocation as before (swift test -c release).

[d-ronnqvist]

@swift-ci please test

[compnerd]

Please do a cross-repository test in swiftlang/swift to verify that this should be safe to merge now.

[d-ronnqvist]

Toolchain builds for all platforms passed in swiftlang/swift#77984