Update PackageIdentity to be case-insensitive

[mattt]

This PR replaces the synthesized conformance to Equatable and Hashable and updates adoption of Comparable to use case-insensitive comparisons.

Motivation:

With SE-0292, package identifiers must be case-insensitive. For example, a dependency declared as mona.LinkedList is equivalent to MONA.LINKEDLIST.

This change also makes sense for packages identified by a remote URL^* or local path^**.

* Technically, URLs are case-sensitive. However, many if not most websites treat them as case-insensitive. This includes all of the popular code hosting services I'm aware of. I would be very surprised if any packages in the wild rely on remote packages differing only in case not being equivalent.

** Filesystems vary in case-sensitivity, so we should to normalize to a single case to make sure Swift packages behave the same on all platforms and don't cause vulnerabilities.

Modifications:

+ extension PackageIdentity: Equatable, Comparable {
+     private func compare(to other: PackageIdentity) -> ComparisonResult {
+         return self.description.caseInsensitiveCompare(other.description)
+     }
+ 
+     public static func == (lhs: PackageIdentity, rhs: PackageIdentity) -> Bool {
+         return lhs.compare(to: rhs) == .orderedSame
+     }
+ 
+     public static func < (lhs: PackageIdentity, rhs: PackageIdentity) -> Bool {
+         return lhs.compare(to: rhs) == .orderedAscending
+     }
+ 
+     public static func > (lhs: PackageIdentity, rhs: PackageIdentity) -> Bool {
+         return lhs.compare(to: rhs) == .orderedDescending
+     }
+ }

Result:

This shouldn't change any existing behavior. If it causes any new failures in our test suite, we should review to see if the expected behavior is correct.

[mattt]

@swift-ci Please smoke test