FileList should conform to Collection (#6845)

It's a little perverse that I can't even ask if the `FileList`
`isEmpty`—currently I'm doing `.contains(where: {_ in true})` and
`RandomAccessCollection` conformance is even simpler to achieve than
`Sequence` conformance. If you really don't want to expose
`RandomAccessCollection` conformance to preserve your ability to change
the representation arbitrarily, at *least* make it conform to
`Collection`, since it is multi-pass.

Author: dabrahams

Sources/PackagePlugin/PackageModel.swift

@@ -418,6 +418,13 @@ extension FileList: Sequence {
     }
 }
 
+@available(_PackageDescription, introduced: 5.10)
+extension FileList: RandomAccessCollection {
+    public var startIndex: Int { 0 }
+    public var endIndex: Int { files.endIndex }
+    public subscript(i: Int) -> File { files[i] }
+}
+
 /// Provides information about a single file in a FileList.
 public struct File {
     /// The path of the file.