Order package manifest to start with the package declaration

Instead of starting with environment variable parsing, start with the actual package manifest, which is most likely the most interesting to most users.

rdar://121441220

Author: ahoppen

Package.swift

@@ -3,62 +3,6 @@
 import Foundation
 import PackageDescription
 
-// MARK: - Parse build arguments
-
-func hasEnvironmentVariable(_ name: String) -> Bool {
-  return ProcessInfo.processInfo.environment[name] != nil
-}
-
-/// Set when building swift-syntax using swift-syntax-dev-utils or in Swift CI in general.
-///
-/// Modifies the build in the following ways
-///  - Enables assertions even in release builds
-///  - Removes the dependency of swift-syntax on os_log
-let buildScriptEnvironment = hasEnvironmentVariable("SWIFT_BUILD_SCRIPT_ENVIRONMENT")
-
-/// Check that the layout of the syntax tree is correct.
-///
-/// See CONTRIBUTING.md for more information
-let rawSyntaxValidation = hasEnvironmentVariable("SWIFTSYNTAX_ENABLE_RAWSYNTAX_VALIDATION")
-
-/// Mutate the input of `assertParse` test cases.
-///
-/// See CONTRIBUTING.md for more information
-let alternateTokenIntrospection = hasEnvironmentVariable("SWIFTPARSER_ENABLE_ALTERNATE_TOKEN_INTROSPECTION")
-
-/// Assume that swift-argument-parser is checked out next to swift-syntax and use that instead of fetching a remote dependency.
-let useLocalDependencies = hasEnvironmentVariable("SWIFTCI_USE_LOCAL_DEPS")
-
-// MARK: - Compute custom build settings
-
-// These build settings apply to the target and the corresponding test target.
-var swiftSyntaxSwiftSettings: [SwiftSetting] = []
-var swiftSyntaxBuilderSwiftSettings: [SwiftSetting] = []
-var swiftParserSwiftSettings: [SwiftSetting] = []
-
-if buildScriptEnvironment {
-  swiftSyntaxSwiftSettings += [
-    .define("SWIFTSYNTAX_ENABLE_ASSERTIONS")
-  ]
-  swiftSyntaxBuilderSwiftSettings += [
-    .define("SWIFTSYNTAX_NO_OSLOG_DEPENDENCY")
-  ]
-}
-
-if rawSyntaxValidation {
-  swiftSyntaxSwiftSettings += [
-    .define("SWIFTSYNTAX_ENABLE_RAWSYNTAX_VALIDATION")
-  ]
-}
-
-if alternateTokenIntrospection {
-  swiftParserSwiftSettings += [
-    .define("SWIFTPARSER_ENABLE_ALTERNATE_TOKEN_INTROSPECTION")
-  ]
-}
-
-// MARK: - Build the package
-
 let package = Package(
   name: "swift-syntax",
   platforms: [
@@ -335,3 +279,57 @@ package.targets.append(
     }
   )
 )
+
+// MARK: - Parse build arguments
+
+func hasEnvironmentVariable(_ name: String) -> Bool {
+  return ProcessInfo.processInfo.environment[name] != nil
+}
+
+/// Set when building swift-syntax using swift-syntax-dev-utils or in Swift CI in general.
+///
+/// Modifies the build in the following ways
+///  - Enables assertions even in release builds
+///  - Removes the dependency of swift-syntax on os_log
+var buildScriptEnvironment: Bool { hasEnvironmentVariable("SWIFT_BUILD_SCRIPT_ENVIRONMENT") }
+
+/// Check that the layout of the syntax tree is correct.
+///
+/// See CONTRIBUTING.md for more information
+var rawSyntaxValidation: Bool { hasEnvironmentVariable("SWIFTSYNTAX_ENABLE_RAWSYNTAX_VALIDATION") }
+
+/// Mutate the input of `assertParse` test cases.
+///
+/// See CONTRIBUTING.md for more information
+var alternateTokenIntrospection: Bool { hasEnvironmentVariable("SWIFTPARSER_ENABLE_ALTERNATE_TOKEN_INTROSPECTION") }
+
+/// Assume that swift-argument-parser is checked out next to swift-syntax and use that instead of fetching a remote dependency.
+var useLocalDependencies: Bool { hasEnvironmentVariable("SWIFTCI_USE_LOCAL_DEPS") }
+
+// MARK: - Compute custom build settings
+
+// These build settings apply to the target and the corresponding test target.
+var swiftSyntaxSwiftSettings: [SwiftSetting] {
+  var settings: [SwiftSetting] = []
+  if buildScriptEnvironment {
+    settings.append(.define("SWIFTSYNTAX_ENABLE_ASSERTIONS"))
+  }
+  if rawSyntaxValidation {
+    settings.append(.define("SWIFTSYNTAX_ENABLE_RAWSYNTAX_VALIDATION"))
+  }
+  return settings
+}
+var swiftSyntaxBuilderSwiftSettings: [SwiftSetting] {
+  if buildScriptEnvironment {
+    return [.define("SWIFTSYNTAX_NO_OSLOG_DEPENDENCY")]
+  } else {
+    return []
+  }
+}
+var swiftParserSwiftSettings: [SwiftSetting] {
+  if alternateTokenIntrospection {
+    return [.define("SWIFTPARSER_ENABLE_ALTERNATE_TOKEN_INTROSPECTION")]
+  } else {
+    return []
+  }
+}