Stop checking in Package.resolved (#1008)

Motivation:

Package.resolved offers little benefit when checked in for libraries.
Let's remove it.

Modifications:

- gitignore, and delete Package.resolved
- Add a Makefile target to build it when we need it

Result:

No Package.resolved checked in

.gitignore

@@ -19,3 +19,4 @@ Examples/EchoWeb/node_modules
 Examples/EchoWeb/package-lock.json
 dev/codegen-tests/**/generated/*
 /scripts/.swiftformat-source/
+Package.resolved

Makefile

@@ -24,6 +24,9 @@ XCODEPROJ:=GRPC.xcodeproj
 all:
 	${SWIFT_BUILD}
 
+Package.resolved:
+	${SWIFT_PACKAGE} resolve
+
 .PHONY:
 plugins: ${PROTOC_GEN_SWIFT} ${PROTOC_GEN_GRPC_SWIFT}
 	cp $^ .

Package.resolved

@@ -1,61 +0,0 @@
-{
-  "object": {
-    "pins": [
-      {
-        "package": "swift-log",
-        "repositoryURL": "https://github.com/apple/swift-log.git",
-        "state": {
-          "branch": null,
-          "revision": "173f567a2dfec11d74588eea82cecea555bdc0bc",
-          "version": "1.4.0"
-        }
-      },
-      {
-        "package": "swift-nio",
-        "repositoryURL": "https://github.com/apple/swift-nio.git",
-        "state": {
-          "branch": null,
-          "revision": "5fc24345f92ec4c274121776c215ab0aa1ed4d50",
-          "version": "2.22.0"
-        }
-      },
-      {
-        "package": "swift-nio-http2",
-        "repositoryURL": "https://github.com/apple/swift-nio-http2.git",
-        "state": {
-          "branch": null,
-          "revision": "1e68e51752be0b43c5a0ef35818c1dd24d13e77c",
-          "version": "1.14.1"
-        }
-      },
-      {
-        "package": "swift-nio-ssl",
-        "repositoryURL": "https://github.com/apple/swift-nio-ssl.git",
-        "state": {
-          "branch": null,
-          "revision": "d381bc53edd9de88a75480a2b969bfc26d61ee76",
-          "version": "2.8.0"
-        }
-      },
-      {
-        "package": "swift-nio-transport-services",
-        "repositoryURL": "https://github.com/apple/swift-nio-transport-services.git",
-        "state": {
-          "branch": null,
-          "revision": "2ac8fde712c1b1a147ecb7065824a40d2c09d0cb",
-          "version": "1.6.0"
-        }
-      },
-      {
-        "package": "SwiftProtobuf",
-        "repositoryURL": "https://github.com/apple/swift-protobuf.git",
-        "state": {
-          "branch": null,
-          "revision": "7f36441e3372665b1b414f8ac93b5905cc42a405",
-          "version": "1.9.0"
-        }
-      }
-    ]
-  },
-  "version": 1
-}