Add target ConnectionPoolModule (vapor#412)

Add `ConnectionPoolModule`

We want to land a new ConnectionPool into PostgresNIO in the comming weeks. Since this pool is abstract, let's create a target and product for it. The target and product are both underscored, to signal that we don't make any API stability guarantees.

Author: fabianfett

Diff for: Package.swift

@@ -11,9 +11,11 @@ let package = Package(
     ],
     products: [
         .library(name: "PostgresNIO", targets: ["PostgresNIO"]),
+        .library(name: "_ConnectionPoolModule", targets: ["_ConnectionPoolModule"]),
     ],
     dependencies: [
         .package(url: "https://github.com/apple/swift-atomics.git", from: "1.2.0"),
+        .package(url: "https://github.com/apple/swift-collections.git", from: "1.0.4"),
         .package(url: "https://github.com/apple/swift-nio.git", from: "2.59.0"),
         .package(url: "https://github.com/apple/swift-nio-transport-services.git", from: "1.19.0"),
         .package(url: "https://github.com/apple/swift-nio-ssl.git", from: "2.25.0"),
@@ -25,6 +27,7 @@ let package = Package(
         .target(
             name: "PostgresNIO",
             dependencies: [
+                .target(name: "_ConnectionPoolModule"),
                 .product(name: "Atomics", package: "swift-atomics"),
                 .product(name: "Crypto", package: "swift-crypto"),
                 .product(name: "Logging", package: "swift-log"),
@@ -38,6 +41,14 @@ let package = Package(
                 .product(name: "NIOFoundationCompat", package: "swift-nio"),
             ]
         ),
+        .target(
+            name: "_ConnectionPoolModule",
+            dependencies: [
+                .product(name: "Atomics", package: "swift-atomics"),
+                .product(name: "DequeModule", package: "swift-collections"),
+            ],
+            path: "Sources/ConnectionPoolModule"
+        ),
         .testTarget(
             name: "PostgresNIOTests",
             dependencies: [
@@ -46,6 +57,16 @@ let package = Package(
                 .product(name: "NIOTestUtils", package: "swift-nio"),
             ]
         ),
+        .testTarget(
+            name: "ConnectionPoolModuleTests",
+            dependencies: [
+                .target(name: "_ConnectionPoolModule"),
+                .product(name: "DequeModule", package: "swift-collections"),
+                .product(name: "NIOCore", package: "swift-nio"),
+                .product(name: "NIOConcurrencyHelpers", package: "swift-nio"),
+                .product(name: "NIOEmbedded", package: "swift-nio"),
+            ]
+        ),
         .testTarget(
             name: "IntegrationTests",
             dependencies: [

Diff for: Sources/ConnectionPoolModule/gitkeep.swift

@@ -0,0 +1 @@
+

Diff for: Tests/ConnectionPoolModuleTests/gitkeep.swift

@@ -0,0 +1 @@
+