Use FoundationEssentials where possible

Motivation:

FoundationEssentials only includes ... the essentials. We should use it
where available.

Modifications:

- Remove unused Foundation imports
- Replace a Foundation import with a FoundationEssentials import

Result:

Smaller dependency set

Sources/GRPCProtobufCodeGen/ProtobufCodeGenParser.swift

@@ -14,7 +14,6 @@
  * limitations under the License.
  */
 
-internal import Foundation
 internal import SwiftProtobuf
 package import SwiftProtobufPluginLibrary
 
@@ -25,6 +24,12 @@ package import struct GRPCCodeGen.Name
 package import struct GRPCCodeGen.ServiceDescriptor
 package import struct GRPCCodeGen.SourceGenerator
 
+#if canImport(FoundationEssentials)
+internal import struct FoundationEssentials.IndexPath
+#else
+internal import struct Foundation.IndexPath
+#endif
+
 /// Parses a ``FileDescriptor`` object into a ``CodeGenerationRequest`` object.
 package struct ProtobufCodeGenParser {
   let extraModuleImports: [String]

Sources/protoc-gen-grpc-swift/GenerateGRPC.swift

@@ -14,7 +14,6 @@
  * limitations under the License.
  */
 
-import Foundation
 import GRPCCodeGen
 import GRPCProtobufCodeGen
 import SwiftProtobuf

Sources/protoc-gen-grpc-swift/Options.swift

@@ -13,7 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-import Foundation
+
 import SwiftProtobufPluginLibrary
 
 enum GenerationError: Error {