swift_package_product_dependency: override `ascii_plist_annotation`.

[GalBerezansky]

We have a build plugin which is a Swift package, it's represented as a XCSwiftPackageProductDependency object. The display name contains a "plugin:" prefix so when it's serialized it's everywhere. The problem is that Xcode removes this prefix and uses the package name in the keys of the serialized objects and in refs to them.

Below is an example before and after the change.

before (wrong):

/* Begin XCSwiftPackageProductDependency section */
        19AA76E52A9F557900F1F6A1 /* plugin:SomePackageName */ = {
            isa = XCSwiftPackageProductDependency;
            productName = "plugin:SomePackageName";
        };

after (correct):

/* Begin XCSwiftPackageProductDependency section */
        19AA76E52A9F557900F1F6A1 /* SomePackageName */ = {
            isa = XCSwiftPackageProductDependency;
            productName = "plugin:SomePackageName";
        };

[GalBerezansky]

The .delete_prefix('plugin:') is a little meh but I couldn't find a better solution for this issue.

[GalBerezansky]

@dnkoutso Can you take a look please? Thank you 😄