Add support for the Swift package index (#297)

# Motivation
We had some scripts around to generate docs for this package but all of our other packages are now providing documentation on the Swift package index.

# Modification
This removes the old script to generate documentation and adds an `.spi.yml` file. Furthermore, I removed the conditional dependency on the docc plugin which we are unconditionally depending on in most of our other packages like NIO.

Author: FranzBusch

Diff for: .spi.yml

@@ -0,0 +1,4 @@
+version: 1
+builder:
+  configs:
+    - documentation_targets: [AsyncAlgorithms]

Diff for: Package.swift

@@ -13,7 +13,10 @@ let package = Package(
   products: [
     .library(name: "AsyncAlgorithms", targets: ["AsyncAlgorithms"]),
   ],
-  dependencies: [.package(url: "https://github.com/apple/swift-collections.git", .upToNextMajor(from: "1.0.4"))],
+  dependencies: [
+    .package(url: "https://github.com/apple/swift-collections.git", from: "1.0.4"),
+    .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
+  ],
   targets: [
     .target(
       name: "AsyncAlgorithms",
@@ -46,18 +49,3 @@ let package = Package(
     ),
   ]
 )
-
-#if canImport(Darwin)
-import Darwin
-let buildingDocs = getenv("BUILDING_FOR_DOCUMENTATION_GENERATION") != nil
-#elseif canImport(Glibc)
-import Glibc
-let buildingDocs = getenv("BUILDING_FOR_DOCUMENTATION_GENERATION") != nil
-#else
-let buildingDocs = false
-#endif
-
-// Only require the docc plugin when building documentation
-package.dependencies += buildingDocs ? [
-  .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
-] : []

Diff for: Package@swift-5.7.swift

@@ -16,7 +16,10 @@ let package = Package(
     .library(name: "_CAsyncSequenceValidationSupport", type: .static, targets: ["AsyncSequenceValidation"]),
     .library(name: "AsyncAlgorithms_XCTest", targets: ["AsyncAlgorithms_XCTest"]),
   ],
-  dependencies: [.package(url: "https://github.com/apple/swift-collections.git", .upToNextMajor(from: "1.0.4"))],
+  dependencies: [
+    .package(url: "https://github.com/apple/swift-collections.git", from: "1.0.4"),
+    .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
+  ],
   targets: [
     .target(
       name: "AsyncAlgorithms",
@@ -34,18 +37,3 @@ let package = Package(
       dependencies: ["AsyncAlgorithms", "AsyncSequenceValidation", "AsyncAlgorithms_XCTest"]),
   ]
 )
-
-#if canImport(Darwin)
-import Darwin
-let buildingDocs = getenv("BUILDING_FOR_DOCUMENTATION_GENERATION") != nil
-#elseif canImport(Glibc)
-import Glibc
-let buildingDocs = getenv("BUILDING_FOR_DOCUMENTATION_GENERATION") != nil
-#else
-let buildingDocs = false
-#endif
-
-// Only require the docc plugin when building documentation
-package.dependencies += buildingDocs ? [
-  .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
-] : []