cmd/go: add ImportComment to Package struct

It seems reasonable that people might want to look up the
ImportComment with "go list".

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/143600043

src/cmd/go/doc.go

@@ -387,6 +387,7 @@ syntax of package template.  The default output is equivalent to -f
     type Package struct {
         Dir           string // directory containing package sources
         ImportPath    string // import path of package in dir
+        ImportComment string // path in import comment on package statement
         Name          string // package name
         Doc           string // package documentation string
         Target        string // install path

src/cmd/go/list.go

@@ -32,6 +32,7 @@ syntax of package template.  The default output is equivalent to -f
     type Package struct {
         Dir           string // directory containing package sources
         ImportPath    string // import path of package in dir
+        ImportComment string // path in import comment on package statement
         Name          string // package name
         Doc           string // package documentation string
         Target        string // install path

src/cmd/go/pkg.go

@@ -28,6 +28,7 @@ type Package struct {
 	// remove existing ones.  Keep in sync with list.go
 	Dir           string `json:",omitempty"` // directory containing package sources
 	ImportPath    string `json:",omitempty"` // import path of package in dir
+	ImportComment string `json:",omitempty"` // path in import comment on package statement
 	Name          string `json:",omitempty"` // package name
 	Doc           string `json:",omitempty"` // package documentation string
 	Target        string `json:",omitempty"` // install path
@@ -104,6 +105,7 @@ func (p *Package) copyBuild(pp *build.Package) {
 
 	p.Dir = pp.Dir
 	p.ImportPath = pp.ImportPath
+	p.ImportComment = pp.ImportComment
 	p.Name = pp.Name
 	p.Doc = pp.Doc
 	p.Root = pp.Root