golang: mod and list commands completion (#7665)

Also, add `gom` alias to `go mod`.

plugins/golang/golang.plugin.zsh

@@ -28,6 +28,7 @@ __go_tool_complete() {
     'help[display help]'
     'install[compile and install packages and dependencies]'
     'list[list packages]'
+    'mod[modules maintenance]'
     'run[compile and run Go program]'
     'test[test packages]'
     'tool[run specified go tool]'
@@ -83,7 +84,7 @@ __go_tool_complete() {
       "-x[print remove commands as it executes them]" \
       "*:importpaths:__go_packages"
       ;;
-  fix|fmt|list|vet)
+  fix|fmt|vet)
       _alternative ':importpaths:__go_packages' ':files:_path_files -g "*.go"'
       ;;
   install)
@@ -124,6 +125,81 @@ __go_tool_complete() {
         "-memprofilerate[set heap profiling rate]:number" \
         "*:args:{ _alternative ':importpaths:__go_packages' ':files:_path_files -g \"*.go\"' }"
       ;;
+  list)
+      _arguments -s -w : \
+        "-f[alternative format for the list]:format" \
+        "-json[print data in json format]" \
+        "-compiled[set CompiledGoFiles to the Go source files presented to the compiler]" \
+        "-deps[iterate over not just the named packages but also all their dependencies]" \
+        "-e[change the handling of erroneous packages]" \
+        "-export[set the Export field to the name of a file containing up-to-date export information for the given package]" \
+        "-find[identify the named packages but not resolve their dependencies]" \
+        "-test[report not only the named packages but also their test binaries]" \
+        "-m[list modules instead of packages]" \
+        "-u[adds information about available upgrades]" \
+        "-versions[set the Module's Versions field to a list of all known versions of that module]:number" \
+        "*:importpaths:__go_packages"
+      ;;
+  mod)
+      typeset -a mod_commands
+      mod_commands+=(
+        'download[download modules to local cache]'
+        'edit[edit go.mod from tools or scripts]'
+        'graph[print module requirement graph]'
+        'init[initialize new module in current directory]'
+        'tidy[add missing and remove unused modules]'
+        'vendor[make vendored copy of dependencies]'
+        'verify[verify dependencies have expected content]'
+        'why[explain why packages or modules are needed]'
+      )
+      if (( CURRENT == 3 )); then
+          _values 'go mod commands' ${mod_commands[@]} "help[display help]"
+          return
+      fi
+      case ${words[3]} in
+      help)
+        _values 'go mod commands' ${mod_commands[@]}
+        ;;
+      download)
+        _arguments -s -w : \
+          "-json[print a sequence of JSON objects standard output]" \
+          "*:flags"
+        ;;
+      edit)
+        _arguments -s -w : \
+          "-fmt[reformat the go.mod file]" \
+          "-module[change the module's path]" \
+          "-replace[=old{@v}=new{@v} add a replacement of the given module path and version pair]:name" \
+          "-dropreplace[=old{@v}=new{@v} drop a replacement of the given module path and version pair]:name" \
+          "-go[={version} set the expected Go language version]:number" \
+          "-print[print the final go.mod in its text format]" \
+          "-json[print the final go.mod file in JSON format]" \
+          "*:flags"
+        ;;
+      graph)
+        ;;
+      init)   
+        ;;
+      tidy)
+        _arguments -s -w : \
+          "-v[print information about removed modules]" \
+          "*:flags"
+        ;;
+      vendor)
+        _arguments -s -w : \
+          "-v[print the names of vendored]" \
+          "*:flags"
+        ;;
+      verify)
+        ;;
+      why)
+        _arguments -s -w : \
+          "-m[treats the arguments as a list of modules and finds a path to any package in each of the modules]" \
+          "-vendor[exclude tests of dependencies]" \
+          "*:importpaths:__go_packages"
+        ;;
+      esac
+      ;;
   help)
       _values "${commands[@]}" \
         'environment[show Go environment variables available]' \
@@ -189,6 +265,7 @@ alias gofa='go fmt ./...'
 alias gog='go get'
 alias goi='go install'
 alias gol='go list'
+alias gom='go mod'
 alias gop='cd $GOPATH'
 alias gopb='cd $GOPATH/bin'
 alias gops='cd $GOPATH/src'