Bug fixes and update latest version of gopls to be 0.3.1

• Ilya Danilkin (@nezorflame)

  • Use v2 of the gopkgs tool to avoid the error in tool installation. Fixes Bug 3050 with PR 3057

• Hyang-Ah Hana Kim (@hyangah)

  • Stop tools listed in the go.alternateTools setting from appearing in the drop down resulting
    from the Go: Install/Update Tools command. Fixes Bug 3024 with PR 3046

• Ramya Rao (@ramya-rao-a)

  • Ensure users on versions older than 0.3.1 of gopls get prompted to update the language server.