Clean up rpaths of installed swift-format

- Don’t explicitly add rpaths to `/usr/lib/swift`, `@executable_path/../lib/swift/macosx` and `@executable_path/../lib/swift-5.5/macosx` on Darwin. I don’t know what they were needed for but they don’t seem to be necessary. Also standard Swift command line tools created from Xcode don’t contain these rpaths.
- Increase the deployment target to macOS 12.0. This removes an rpath in the built binary that is an absolute path to the toolchain on the host, which was used to build swift-format.

rdar://121400644

Author: ahoppen

Package.swift

@@ -35,8 +35,8 @@ if installAction {
 let package = Package(
   name: "swift-format",
   platforms: [
-    .iOS("13.0"),
-    .macOS("10.15")
+    .macOS("12.0"),
+    .iOS("13.0")
   ],
   products: [
     .executable(

build-script-helper.py

@@ -110,26 +110,7 @@ def get_swiftpm_options(
         swift_exec, cross_compile_config=cross_compile_config
     )
     build_os = build_target.split("-")[2]
-    if build_os.startswith("macosx"):
-        args += [
-            "-Xlinker",
-            "-rpath",
-            "-Xlinker",
-            "/usr/lib/swift",
-        ]
-        args += [
-            "-Xlinker",
-            "-rpath",
-            "-Xlinker",
-            "@executable_path/../lib/swift/macosx",
-        ]
-        args += [
-            "-Xlinker",
-            "-rpath",
-            "-Xlinker",
-            "@executable_path/../lib/swift-5.5/macosx",
-        ]
-    else:
+    if not build_os.startswith("macosx"):
         # Library rpath for swift, dispatch, Foundation, etc. when installing
         args += [
             "-Xlinker",