refactor: rename cache package from v4 to v2

Rename the cache concurrent map package from v4 to v2 to better reflect
the actual version or align with project versioning strategy.

Changes:
- Move pkg/cache/v4/cmap.go to pkg/cache/v2/cmap.go
- Update package declaration from v4 to cachev2
- Update import alias from cachev4 to cachev2 in backend/inmemory.go
- Update all references to use the new package name

This change maintains all existing functionality while correcting the
package versioning to match the intended version number.

Author: hyp3rd

backend/inmemory.go

@@ -7,22 +7,22 @@ import (
 	"github.com/hyp3rd/hypercache/internal/constants"
 	"github.com/hyp3rd/hypercache/internal/sentinel"
 	"github.com/hyp3rd/hypercache/pkg/cache"
-	cachev4 "github.com/hyp3rd/hypercache/pkg/cache/v4"
+	cachev2 "github.com/hyp3rd/hypercache/pkg/cache/v2"
 )
 
 // InMemory is a cache backend that stores the items in memory, leveraging a custom `ConcurrentMap`.
 type InMemory struct {
 	sync.RWMutex // mutex to protect the cache from concurrent access
 
-	items           cachev4.ConcurrentMap  // map to store the items in the cache
+	items           cachev2.ConcurrentMap  // map to store the items in the cache
 	itemPoolManager *cache.ItemPoolManager // item pool manager to manage the item pool
 	capacity        int                    // capacity of the cache, limits the number of items that can be stored in the cache
 }
 
 // NewInMemory creates a new in-memory cache with the given options.
 func NewInMemory(opts ...Option[InMemory]) (IBackend[InMemory], error) {
 	InMemory := &InMemory{
-		items:           cachev4.New(),
+		items:           cachev2.New(),
 		itemPoolManager: cache.NewItemPoolManager(),
 	}
 	// Apply the backend options

pkg/cache/v4/cmap.go pkg/cache/v2/cmap.gopkg/cache/v4/cmap.go renamed to pkg/cache/v2/cmap.go

@@ -19,7 +19,7 @@
 //	if item, ok := cm.Get("key"); ok {
 //	    // Process item
 //	}
-package v4
+package cachev2
 
 import (
 	"hash"