Add standard SwiftPM versioning support (#6860)

Author: paulb777

Package.swift

@@ -100,19 +100,17 @@ let package = Package(
     ),
     .package(
       name: "nanopb",
-      url: "https://github.com/nanopb/nanopb.git",
+      url: "https://github.com/firebase/nanopb.git",
       // This revision adds SPM enablement to the 0.3.9.6 release tag.
-      .revision("8119dfe5631f2616d11e50ead95448d12e816062")
-    ),
-    .package(
-      name: "abseil",
-      url: "https://github.com/firebase/abseil-cpp-SwiftPM.git",
-      .revision("05d8107f2971a37e6c77245b7c4c6b0a7e97bc99")
+      "2.30906.0" ..< "2.30907.0"
     ),
+    .package(name: "abseil",
+             url: "https://github.com/firebase/abseil-cpp-SwiftPM.git",
+             from: "0.20200225.0"),
     .package(
       name: "gRPC",
       url: "https://github.com/firebase/grpc-SwiftPM.git",
-      .revision("5bb2669317ae2183f4cb00c675423af1924f0b46")
+      "1.28.2" ..< "1.29.0"
     ),
     .package(
       name: "OCMock",
@@ -122,7 +120,7 @@ let package = Package(
     .package(
       name: "leveldb",
       url: "https://github.com/firebase/leveldb.git",
-      .revision("fa1f25f296a766e5a789c4dacd4798dea798b2c2")
+      "1.22.1" ..< "1.23.0"
     ),
     // Branches need a force update with a run with the revision set like below.
     //   .package(url: "https://github.com/paulb777/nanopb.git", .revision("564392bd87bd093c308a3aaed3997466efb95f74"))

SwiftPackageManager.md

@@ -28,12 +28,13 @@ Select the Firebase GitHub repository - `https://github.com/firebase/firebase-io
 
 <img src="docs/resources/SPMChoose.png">
 
-Select the beta branch.
+Select the version.
 
-Note: Starting with the 6.31.0 release, the versions are specified
-in a format like 6.34-spm-beta. We won't support standard repository
-versioning until later in the beta or with general availability of the SPM
-distribution.
+Note: Prior to the 7.0.0 release, version selection is not available. Instead, the releases
+are available in branches with names like 6.34-spm-beta.
+
+Note: The Swift Package Manager distribution continues to be in beta even though it now
+supports standard Swift Package Manager versioning.
 
 <img src="docs/resources/SPMSelect.png">