[benchmark] Refactor out how we specify benchmarks in Package.swift so downstream we can add additional source folders.

This is just a useful thing if one wants to have a separate folder of down
stream benchmarks. The code is written like this to hopefully prevent merge
conflicts from happening.

Author: gottesmm

benchmark/Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:4.2
+// swift-tools-version:5.0
 
 import PackageDescription
 import Foundation
@@ -9,10 +9,15 @@ unsupportedTests.insert("ObjectiveCBridging")
 unsupportedTests.insert("ObjectiveCBridgingStubs")
 #endif
 
-// This is a stop gap hack so we can edit benchmarks in Xcode.
-let singleSourceLibraries: [String] = {
+//===---
+// Single Source Libraries
+//
+
+/// Return the source files in subDirectory that we will translate into
+/// libraries. Each source library will be compiled as its own module.
+func getSingleSourceLibraries(subDirectory: String) -> [String] {
   let f = FileManager.`default`
-  let dirURL = URL(fileURLWithPath: "single-source").absoluteURL
+  let dirURL = URL(fileURLWithPath: subDirectory)
   let fileURLs = try! f.contentsOfDirectory(at: dirURL,
                                             includingPropertiesForKeys: nil)
   return fileURLs.compactMap { (path: URL) -> String? in
@@ -25,27 +30,45 @@ let singleSourceLibraries: [String] = {
       return nil
     }
 
-    let s = String(c[0])
+    let name = String(c[0])
 
     // We do not support this test.
-    if unsupportedTests.contains(s) {
+    if unsupportedTests.contains(name) {
       return nil
     }
 
-    assert(s != "PrimsSplit")
-    return s
+    return name
   }
-}()
+}
+
+var singleSourceLibraryDirs: [String] = []
+singleSourceLibraryDirs.append("single-source")
+
+var singleSourceLibraries: [String] = singleSourceLibraryDirs.flatMap {
+  getSingleSourceLibraries(subDirectory: $0)
+}
 
-let multiSourceLibraries: [String] = {
+//===---
+// Multi Source Libraries
+//
+
+func getMultiSourceLibraries(subDirectory: String) -> [(String, String)] {
   let f = FileManager.`default`
-  let dirURL = URL(fileURLWithPath: "multi-source").absoluteURL
-  let fileURLs = try! f.contentsOfDirectory(at: dirURL,
-                                            includingPropertiesForKeys: nil)
-  return fileURLs.map { (path: URL) -> String in
-    return path.lastPathComponent
-  }
-}()
+  let dirURL = URL(string: subDirectory)!
+  let subDirs = try! f.contentsOfDirectory(at: dirURL, includingPropertiesForKeys: nil)
+  return subDirs.map { (subDirectory, $0.lastPathComponent) }
+}
+
+var multiSourceLibraryDirs: [String] = []
+multiSourceLibraryDirs.append("multi-source")
+
+var multiSourceLibraries: [(parentSubDir: String, name: String)] = multiSourceLibraryDirs.flatMap {
+  getMultiSourceLibraries(subDirectory: $0)
+}
+
+//===---
+// Products
+//
 
 var products: [Product] = []
 products.append(.library(name: "TestsUtils", type: .static, targets: ["TestsUtils"]))
@@ -54,9 +77,15 @@ products.append(.library(name: "DriverUtils", type: .static, targets: ["DriverUt
 products.append(.library(name: "ObjectiveCTests", type: .static, targets: ["ObjectiveCTests"]))
 #endif
 products.append(.executable(name: "SwiftBench", targets: ["SwiftBench"]))
-products.append(.library(name: "PrimsSplit", type: .static, targets: ["PrimsSplit"]))
+
 products += singleSourceLibraries.map { .library(name: $0, type: .static, targets: [$0]) }
-products += multiSourceLibraries.map { .library(name: $0, type: .static, targets: [$0]) }
+products += multiSourceLibraries.map {
+  return .library(name: $0.name, type: .static, targets: [$0.name])
+}
+
+//===---
+// Targets
+//
 
 var targets: [Target] = []
 targets.append(.target(name: "TestsUtils", path: "utils", sources: ["TestsUtils.swift"]))
@@ -73,7 +102,7 @@ swiftBenchDeps.append(.target(name: "ObjectiveCTests"))
 #endif
 swiftBenchDeps.append(.target(name: "DriverUtils"))
 swiftBenchDeps += singleSourceLibraries.map { .target(name: $0) }
-swiftBenchDeps += multiSourceLibraries.map { .target(name: $0) }
+swiftBenchDeps += multiSourceLibraries.map { .target(name: $0.name) }
 
 targets.append(
     .target(name: "SwiftBench",
@@ -92,20 +121,27 @@ var singleSourceDeps: [Target.Dependency] = [.target(name: "TestsUtils")]
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 singleSourceDeps.append(.target(name: "ObjectiveCTests"))
 #endif
-targets += singleSourceLibraries.map { x in
-    return .target(name: x,
+
+targets += singleSourceLibraries.map { name in
+  return .target(name: name,
       dependencies: singleSourceDeps,
       path: "single-source",
-      sources: ["\(x).swift"])
+      sources: ["\(name).swift"])
 }
-targets += multiSourceLibraries.map { x in
-  return .target(name: x,
+
+targets += multiSourceLibraries.map { lib in
+  return .target(
+    name: lib.name,
     dependencies: [
       .target(name: "TestsUtils")
     ],
-    path: "multi-source/\(x)")
+    path: lib.parentSubDir)
 }
 
+//===---
+// Top Level Definition
+//
+
 let p = Package(
   name: "swiftbench",
   products: products,