Use Shwift library instead of Subprocess

Shwift has a concise API, which makes writing shell code nice and easy.
This is an opinionated decision.

Package.resolved

@@ -1,5 +1,5 @@
 {
-  "originHash" : "fd1e824e418c767633bb79b055a4e84d9c86165746bc881d5d27457ad34b0c20",
+  "originHash" : "0cb2e87817f52021ac25ffee6b27396f6d94e9fd604ca83db7f20a10e65fe6cf",
   "pins" : [
     {
       "identity" : "noora",
@@ -28,6 +28,15 @@
         "version" : "4.2.1"
       }
     },
+    {
+      "identity" : "shwift",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/GeorgeLyon/Shwift",
+      "state" : {
+        "revision" : "d7be04898d094ddce6140cc6a2e9a83fc994b66d",
+        "version" : "3.1.1"
+      }
+    },
     {
       "identity" : "swift-argument-parser",
       "kind" : "remoteSourceControl",
@@ -37,15 +46,6 @@
         "version" : "1.7.0"
       }
     },
-    {
-      "identity" : "swift-async-algorithms",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-async-algorithms",
-      "state" : {
-        "revision" : "9d349bcc328ac3c31ce40e746b5882742a0d1272",
-        "version" : "1.1.3"
-      }
-    },
     {
       "identity" : "swift-atomics",
       "kind" : "remoteSourceControl",
@@ -82,15 +82,6 @@
         "version" : "2.95.0"
       }
     },
-    {
-      "identity" : "swift-subprocess",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/swiftlang/swift-subprocess.git",
-      "state" : {
-        "revision" : "ba5888ad7758cbcbe7abebac37860b1652af2d9c",
-        "version" : "0.3.0"
-      }
-    },
     {
       "identity" : "swift-system",
       "kind" : "remoteSourceControl",