Change executable and tool templates to use main.swift

The recent changes to templates switched these templates away from using `@main` which seems to not be compatible with Xcode's build system. The only non-`@main` way for executables that is supported is naming the primary source file "main.swift", so that is what this PR does.

Author: neonichu

Sources/Workspace/InitPackage.swift

@@ -290,7 +290,12 @@ public final class InitPackage {
           : sources.appending(component: "\(pkgname)")
         try makeDirectories(moduleDir)
 
-        let sourceFileName = "\(typeName).swift"
+        let sourceFileName: String
+        if packageType == .executable || packageType == .tool {
+            sourceFileName = "main.swift"
+        } else {
+            sourceFileName = "\(typeName).swift"
+        }
         let sourceFile = try AbsolutePath(validating: sourceFileName, relativeTo: moduleDir)
 
         let content: String

Tests/CommandsTests/PackageToolTests.swift

@@ -718,7 +718,7 @@ final class PackageToolTests: CommandsTestCase {
             XCTAssertMatch(contents, .prefix("// swift-tools-version:\(version < .v5_4 ? "" : " ")\(versionSpecifier)\n"))
 
             XCTAssertFileExists(manifest)
-            XCTAssertEqual(try fs.getDirectoryContents(path.appending(component: "Sources")), ["Foo.swift"])
+            XCTAssertEqual(try fs.getDirectoryContents(path.appending(component: "Sources")), ["main.swift"])
         }
     }
 
@@ -749,7 +749,7 @@ final class PackageToolTests: CommandsTestCase {
             XCTAssertMatch(contents, .prefix("// swift-tools-version:\(version < .v5_4 ? "" : " ")\(versionSpecifier)\n"))
 
             XCTAssertFileExists(manifest)
-            XCTAssertEqual(try fs.getDirectoryContents(path.appending(component: "Sources")), ["CustomName.swift"])
+            XCTAssertEqual(try fs.getDirectoryContents(path.appending(component: "Sources")), ["main.swift"])
         }
     }