Allow building against a single dynamic swift-syntax library

When `SWIFTSYNTAX_BUILD_DYNAMIC_LIBRARY` is specified, change swift-syntax dependencies to depend on a single `_SwiftSyntaxDynamic` product instead of separate products for each module.

This allows us to build SourceKit-LSP on Windows using SwiftPM without exceeding the maximum symbol limit and thus run SourceKit-LSP tests on Windows.

See swiftlang/sourcekit-lsp#1754 and swiftlang/swift-syntax#2879.

Author: ahoppen

Package.swift

@@ -47,22 +47,22 @@ let package = Package(
 
     .target(
       name: "SwiftFormat",
-      dependencies: omittingExternalDependenciesIfNecessary([
-        .product(name: "Markdown", package: "swift-markdown"),
-        .product(name: "SwiftSyntax", package: "swift-syntax"),
-        .product(name: "SwiftSyntaxBuilder", package: "swift-syntax"),
-        .product(name: "SwiftOperators", package: "swift-syntax"),
-        .product(name: "SwiftParser", package: "swift-syntax"),
-        .product(name: "SwiftParserDiagnostics", package: "swift-syntax"),
-      ]),
+      dependencies: [
+        .product(name: "Markdown", package: "swift-markdown")
+      ]
+        + swiftSyntaxDependencies([
+          "SwiftOperators", "SwiftParser", "SwiftParserDiagnostics", "SwiftSyntax", "SwiftSyntaxBuilder",
+        ]),
       exclude: ["CMakeLists.txt"]
     ),
     .target(
       name: "_SwiftFormatTestSupport",
-      dependencies: omittingExternalDependenciesIfNecessary([
-        "SwiftFormat",
-        .product(name: "SwiftOperators", package: "swift-syntax"),
-      ])
+      dependencies: [
+        "SwiftFormat"
+      ]
+        + swiftSyntaxDependencies([
+          "SwiftOperators", "SwiftParser", "SwiftParserDiagnostics", "SwiftSyntax", "SwiftSyntaxBuilder",
+        ]),
     ),
     .plugin(
       name: "Format Source Code",
@@ -98,41 +98,44 @@ let package = Package(
     ),
     .executableTarget(
       name: "swift-format",
-      dependencies: omittingExternalDependenciesIfNecessary([
+      dependencies: [
         "_SwiftFormatInstructionCounter",
         "SwiftFormat",
         .product(name: "ArgumentParser", package: "swift-argument-parser"),
-        .product(name: "SwiftSyntax", package: "swift-syntax"),
-        .product(name: "SwiftParser", package: "swift-syntax"),
-      ]),
+      ] + swiftSyntaxDependencies(["SwiftParser", "SwiftSyntax"]),
       exclude: ["CMakeLists.txt"],
       linkerSettings: swiftformatLinkSettings
     ),
 
     .testTarget(
       name: "SwiftFormatPerformanceTests",
-      dependencies: omittingExternalDependenciesIfNecessary([
+      dependencies: [
         "SwiftFormat",
         "_SwiftFormatTestSupport",
-        .product(name: "SwiftSyntax", package: "swift-syntax"),
-        .product(name: "SwiftParser", package: "swift-syntax"),
-      ])
+      ] + swiftSyntaxDependencies(["SwiftParser", "SwiftSyntax"]),
     ),
     .testTarget(
       name: "SwiftFormatTests",
-      dependencies: omittingExternalDependenciesIfNecessary([
+      dependencies: [
         "SwiftFormat",
         "_SwiftFormatTestSupport",
         .product(name: "Markdown", package: "swift-markdown"),
-        .product(name: "SwiftOperators", package: "swift-syntax"),
-        .product(name: "SwiftParser", package: "swift-syntax"),
-        .product(name: "SwiftSyntax", package: "swift-syntax"),
-        .product(name: "SwiftSyntaxBuilder", package: "swift-syntax"),
-      ])
+      ]
+        + swiftSyntaxDependencies([
+          "SwiftOperators", "SwiftParser", "SwiftSyntax", "SwiftSyntaxBuilder",
+        ]),
     ),
   ]
 )
 
+func swiftSyntaxDependencies(_ names: [String]) -> [Target.Dependency] {
+  if buildDynamicSwiftSyntaxLibrary {
+    return [.product(name: "_SwiftSyntaxDynamic", package: "swift-syntax")]
+  } else {
+    return names.map { .product(name: $0, package: "swift-syntax") }
+  }
+}
+
 // MARK: - Parse build arguments
 
 func hasEnvironmentVariable(_ name: String) -> Bool {
@@ -147,28 +150,19 @@ var installAction: Bool { hasEnvironmentVariable("SWIFTFORMAT_CI_INSTALL") }
 /// remote dependency.
 var useLocalDependencies: Bool { hasEnvironmentVariable("SWIFTCI_USE_LOCAL_DEPS") }
 
-var omitExternalDependencies: Bool { hasEnvironmentVariable("SWIFTFORMAT_OMIT_EXTERNAL_DEPENDENCIES") }
-
-func omittingExternalDependenciesIfNecessary(
-  _ dependencies: [Target.Dependency]
-) -> [Target.Dependency] {
-  guard omitExternalDependencies else {
-    return dependencies
-  }
-  return dependencies.filter { dependency in
-    if case .productItem(_, let package, _, _) = dependency {
-      return package == nil
-    }
-    return true
-  }
+/// Whether swift-syntax is being built as a single dynamic library instead of as a separate library per module.
+///
+/// This means that the swift-syntax symbols don't need to be statically linked, which alles us to stay below the
+/// maximum number of exported symbols on Windows, in turn allowing us to build sourcekit-lsp using SwiftPM on Windows
+/// and run its tests.
+var buildDynamicSwiftSyntaxLibrary: Bool {
+  hasEnvironmentVariable("SWIFTSYNTAX_BUILD_DYNAMIC_LIBRARY")
 }
 
 // MARK: - Dependencies
 
 var dependencies: [Package.Dependency] {
-  if omitExternalDependencies {
-    return []
-  } else if useLocalDependencies {
+  if useLocalDependencies {
     return [
       .package(path: "../swift-argument-parser"),
       .package(path: "../swift-markdown"),