Encode absolute presentation URL if present

Support for absolute URLs in link summaries was added in 7db36ad.

It added support for absolute URLs when decoding a link summary, but not when encoding it:
https://github.com/swiftlang/swift-docc/blob/cfcd96f0e992af2287661f6a26e8398096f6bc1e/Sources/SwiftDocC/LinkTargets/LinkDestinationSummary.swift#L772-L773
https://github.com/swiftlang/swift-docc/blob/cfcd96f0e992af2287661f6a26e8398096f6bc1e/Sources/SwiftDocC/LinkTargets/LinkDestinationSummary.swift#L728

Both are needed when using an external link resolver, as the external link resolver needs to encode the entity to send to DocC.

Fixes rdar://164628218.

Author: anferbui

Sources/SwiftDocC/LinkTargets/LinkDestinationSummary.swift

@@ -726,7 +726,7 @@ extension LinkDestinationSummary {
         } else {
             try container.encode(kind, forKey: .kind)
         }
-        try container.encode(relativePresentationURL, forKey: .relativePresentationURL)
+        try container.encode(absolutePresentationURL ?? relativePresentationURL, forKey: .relativePresentationURL)
         try container.encode(referenceURL, forKey: .referenceURL)
         try container.encode(title, forKey: .title)
         try container.encodeIfPresent(abstract, forKey: .abstract)

Tests/SwiftDocCTests/Infrastructure/ExternalReferenceResolverTests.swift

@@ -1420,7 +1420,10 @@ class ExternalReferenceResolverTests: XCTestCase {
             """.utf8))
         XCTAssertEqual(externalEntity.relativePresentationURL.absoluteString, "/path/to/something")
         XCTAssertEqual(externalEntity.absolutePresentationURL?.absoluteString, "https://com.example/path/to/something")
-        
+
+        // Test that encoding the link summary preserves the absolute URL
+        try assertRoundTripCoding(externalEntity)
+
         let resolver = Resolver(entityToReturn: externalEntity)
 
         var configuration = DocumentationContext.Configuration()