Remove unnecessary RFC 5321 dependency

Removed unused RFC_5321 dependency from both:
- Package.swift (dependency declaration)
- RFC 5322.swift (import statement)

RFC 5322 doesn't use any RFC 5321 types. Cross-RFC conversions
(RFC_5322 ↔ RFC_5321) are now provided by swift-emailaddress-type
to avoid circular dependencies.

This improves package independence and follows the principle that
each RFC package should be as self-contained as possible.

Author: coenttb

Package.swift

@@ -10,7 +10,6 @@ extension String {
 extension Target.Dependency {
     static var rfc5322: Self { .target(name: .rfc5322) }
     static var rfc1123: Self { .product(name: "RFC_1123", package: "swift-rfc-1123") }
-    static var rfc5321: Self { .product(name: "RFC_5321", package: "swift-rfc-5321") }
 }
 
 let package = Package(
@@ -24,14 +23,12 @@ let package = Package(
     ],
     dependencies: [
         .package(url: "https://github.com/swift-standards/swift-rfc-1123.git", from: "0.0.1"),
-        .package(url: "https://github.com/swift-standards/swift-rfc-5321.git", from: "0.0.1"),
     ],
     targets: [
         .target(
             name: .rfc5322,
             dependencies: [
-                .rfc1123,
-                .rfc5321
+                .rfc1123
             ]
         ),
         .testTarget(

Sources/RFC_5322/RFC 5322.swift

@@ -7,7 +7,6 @@
 
 import Foundation
 import RFC_1123
-import RFC_5321
 import RegexBuilder
 
 public enum RFC_5322 {}
@@ -195,15 +194,6 @@ extension RFC_5322.EmailAddress {
     public var addressValue: String {
         "\(localPart.stringValue)@\(domain.name)"
     }
-
-    /// Convert to RFC 5321 format if possible
-    public func toRFC5321() throws -> RFC_5321.EmailAddress {
-        try RFC_5321.EmailAddress(
-            displayName: displayName,
-            localPart: .init(localPart.stringValue),
-            domain: domain
-        )
-    }
 }
 
 // MARK: - Errors