Clean up dependency graph and imports

Motivation:

AWSLambdaRuntimeCore and MockServer both have dependencies on the NIO
module, but they don't express this in their Package.swift: they
implicitly rely on these dependencies being met by the NIOHTTP1 import.
This was always brittle: it could lead to timing issues in the build
that could lead to it failing.

Modifications:

- Correctly express the dependency on NIO in Package.swift
- Correctly import _NIOConcurrency in files where it's used

Result:

Better and more accurate builds.

Package.swift

@@ -29,6 +29,7 @@ let package = Package(
  .product(name: "Logging", package: "swift-log"),
  .product(name: "Backtrace", package: "swift-backtrace"),
  .product(name: "NIOHTTP1", package: "swift-nio"),
+ .product(name: "NIO", package: "swift-nio"),
  .product(name: "_NIOConcurrency", package: "swift-nio"),
  ]),
  .testTarget(name: "AWSLambdaRuntimeCoreTests", dependencies: [
@@ -51,6 +52,7 @@ let package = Package(
  // for perf testing
  .target(name: "MockServer", dependencies: [
  .product(name: "NIOHTTP1", package: "swift-nio"),
+ .product(name: "NIO", package: "swift-nio"),
  ]),
  .target(name: "StringSample", dependencies: ["AWSLambdaRuntime"]),
  .target(name: "CodableSample", dependencies: ["AWSLambdaRuntime"]),

Sources/AWSLambdaRuntimeCore/LambdaHandler.swift

@@ -14,6 +14,7 @@
 
 import Dispatch
 import NIO
+import _NIOConcurrency
 
 // MARK: - LambdaHandler