Handle paginated registry metadata responses (swiftlang#8219)

In [4.1 List Package
Releases](https://github.com/swiftlang/swift-package-manager/blob/main/Documentation/PackageRegistry/Registry.md#41-list-package-releases)
it is stated that a server may respond with a `Link` header that
contains a pointer to a subsequent page of results. SwiftPM is not
checking for the `next` link in the `Link` header and so if a registry
returns paginated results, only the first page of versions is searched
when resolving. This would result in a "version not found" error when in
reality the version is present in a subsequent page of results.

Respect the `next` link in the `Link` header by loading the next page of
results and building up a list of versions, continuing until there is no
`next` link present in the `Link` header of the last result.

Registry servers that serve paginated results now have all their results
read.

Issue: swiftlang#8215

Author: plemarquand

Sources/PackageMetadata/PackageMetadata.swift

@@ -352,7 +352,7 @@ public struct PackageSearchClient {
         ) { result in
             do {
                 let metadata = try result.get()
-                let alternateLocations = metadata.alternateLocations ?? []
+                let alternateLocations = metadata.alternateLocations
                 return completion(.success(Set(alternateLocations)))
             } catch {
                 return completion(.failure(error))