Comments/cleanup to Package.swift. (#371)

* Comments/cleanup to Package.swift.

Added some comments and deleted two implicit dependencies; let's see if it still compiles without these. Otherwise, I'll add them back.

* Update Package.swift

* Update Package.swift

* Add `Package.resolved` and switch our dependencies to use `.upToNextMajor`.

* Update Package.resolved.

* Use upToNextMinor for Commander.

.gitignore

@@ -12,7 +12,6 @@ third_party/**
 /test.out
 /echo.pid
 /SwiftGRPC.xcodeproj
-Package.resolved
 Examples/EchoWeb/dist
 Examples/EchoWeb/node_modules
 Examples/EchoWeb/package-lock.json