Support Go modules

[itchyny]

This pull request is based on #151, #164 and motemen/go-quickfix#6.

Should fix these all......

• close Support Go modules #143 Support Go modules
  • gore should work with the local module outside GOPATH
  • gore should automatically download the module with :import without go get-ting it.
• import from deep directory in module
• [ ] import the same version as go.mod
• :type
• :doc
• Add tests
• code completion
  • https://github.com/stamblerre/gocode somehow does not work
• close gore can't import vendored packages #72 gore can't import vendored packages
  • The vendor/ directory is not used anymore. gore should support go.mod definition instead (including replace directive)
• close Import fails to find pacakge that exists #68 Import fails to find pacakge that exists
• close Importing local packages #69 Importing local packages
  • maxp-edcast@df5e7c1
• offline support (as much as possible)
  • this is too difficult. only for printer packages
• Performance improvement (packages.Load is too slow)
• All features should work with the value of GO111MODULE
• autoimport
• pkg
• automatically import pretty printer
• close Automatic go get? #83 automatic go get
• close gore can't import vendored packages #72 import the module listed in go.mod
• import specific versions