[#7] engine: Set project structure pattern for files

* Create pkg package
* Move chain-relates structures to pkg/chain package
* Move inmemory and interface files to pkg/engine package
* Move resource structures to pkg/resource package
* Move GlobMatch to util package

Signed-off-by: Airat Arifullin <aarifullin@yadro.com>

pkg/engine/interface.go

@@ -0,0 +1,30 @@
+package engine
+
+import (
+	"git.frostfs.info/TrueCloudLab/policy-engine/pkg/chain"
+	"git.frostfs.info/TrueCloudLab/policy-engine/pkg/resource"
+)
+
+// Engine ...
+type Engine interface {
+	// IsAllowed returns status for the operation after all checks.
+	// The second return value signifies whether a matching rule was found.
+	IsAllowed(name chain.Name, namespace string, r resource.Request) (chain.Status, bool)
+}
+
+// CachedChainStorage ...
+type CachedChainStorage interface {
+	Engine
+	// Adds a policy chain used for all operations with a specific resource.
+	AddResourceChain(name chain.Name, resource string, c *chain.Chain)
+	// Adds a policy chain used for all operations in the namespace.
+	AddNameSpaceChain(name chain.Name, namespace string, c *chain.Chain)
+	// Adds a local policy chain used for all operations with this service.
+	AddOverride(name chain.Name, c *chain.Chain)
+	// Gets the local override with given chain id.
+	GetOverride(name chain.Name, chainID chain.ID) (chain *chain.Chain, found bool)
+	// Remove the local override with given chain id.
+	RemoveOverride(name chain.Name, chainID chain.ID) (removed bool)
+	// ListOverrides returns the list of local overrides.
+	ListOverrides(name chain.Name) []*chain.Chain
+}