Further alignment of preliminary implementation with SE-0303 proposal (#3312)

Move the usage of plugins from the `dependencies` parameter of targets in the package manifest to instead be in a separate `plugins` parameter.

Despite their similarity to dependencies we want users to consider them differently.  This also leaves room to grow in the future, when plugin usage might support parameters, etc.  We encode them separately in the manifest JSON, but merge them into regular dependencies to avoid duplicated implementation on the SwiftPM side.  This can be changed later without affecting clients.  For now it would just mean duplicate code.

rdar://74663667

Author: abertelrud

Fixtures/Miscellaneous/Plugins/MySourceGenClient/Package.swift

@@ -17,8 +17,8 @@ let package = Package(
         // A unit that uses the plugin.
         .testTarget(
             name: "MyTests",
-            dependencies: [
-                .product(name: "MySourceGenPlugin", package: "MySourceGenPlugin")
+            plugins: [
+                .plugin(name: "MySourceGenPlugin", package: "MySourceGenPlugin")
             ]
         )
     ]

Fixtures/Miscellaneous/Plugins/MySourceGenPlugin/Package.swift

@@ -49,8 +49,10 @@ let package = Package(
         .testTarget(
             name: "MySourceGenPluginTests",
             dependencies: [
-                "MySourceGenPlugin",
                 "MySourceGenRuntimeLib"
+            ],
+            plugins: [
+                "MySourceGenPlugin"
             ]
         )
     ]