Factor out resolution responsibilities

Sources/apk/Index/ApkIndexProviderCache.swift

@@ -0,0 +1,37 @@
+/*
+ * darwin-apk © 2025 Gay Pizza Specifications
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import Foundation
+
+public struct ApkIndexProviderCache {
+  private var _providers = [(ApkIndexProvides, ApkIndex.Index)]()
+  //private var _installIfCache = [(ApkIndexInstallIf, Set<ApkIndex.Index>)]()
+
+  public init(index pkgIndex: ApkIndex) {
+    for (index, pkg) in pkgIndex.packages.enumerated() {
+      self._providers.append((.specific(name: pkg.name, version: pkg.version), index))
+      for provision in pkg.provides {
+        self._providers.append((provision, index))
+      }
+      //for installIf in pkg.installIf {
+      //  self._installIfCache.append((installIf, index))
+      //}
+    }
+  }
+}
+
+extension ApkIndexProviderCache {
+  func resolve(index pkgIndex: ApkIndex, requirement: ApkVersionRequirement) -> ApkIndex.Index? {
+    self._providers.filter { prv in prv.0.satisfies(requirement) }
+      .max { pkgIndex.packages[$0.1] < pkgIndex.packages[$1.1] }?.1
+  }
+}
+
+extension ApkIndexPackage: Comparable {
+  public static func < (lhs: Self, rhs: Self) -> Bool {
+    // Prefer highest declared provider priority
+    lhs.providerPriority ?? 0 < rhs.providerPriority ?? 0
+  }
+}