Adds -tags support

This commit is a patch based on tools#117
that adds support for -tags to godep.

It is REQUIRED in order to save the deps for a different platform on
your dev machine; in our case it is to save off the heroku deps
in addition to our application deps in order to pull in and build the
goose migration tool remotely and have it available on heroku.

pkg.go

@@ -32,10 +32,19 @@ type Package struct {
 // Unlike the go tool, an empty argument list is treated as
 // an empty list; "." must be given explicitly if desired.
 func LoadPackages(name ...string) (a []*Package, err error) {
+	args := []string{"list", "-e", "-json"}
+	return runListCommand(args, name...)
+}
+
+func LoadPackagesWithTags(tags string, name ...string) (a []*Package, err error) {
+	args := []string{"list", "-e", "-json", "-tags", tags}
+	return runListCommand(args, name...)
+}
+
+func runListCommand(args []string, name ...string) (a []*Package, err error) {
 	if len(name) == 0 {
 		return nil, nil
 	}
-	args := []string{"list", "-e", "-json"}
 	cmd := exec.Command("go", append(args, name...)...)
 	r, err := cmd.StdoutPipe()
 	if err != nil {

save.go

@@ -17,7 +17,7 @@ import (
 )
 
 var cmdSave = &Command{
-	Usage: "save [-r] [packages]",
+	Usage: "save [-r] [-tags=\"\"] [packages]",
 	Short: "list and copy dependencies into Godeps",
 	Long: `
 Save writes a list of the dependencies of the named packages along
@@ -46,6 +46,9 @@ To update a dependency to a newer revision, use 'godep update'.
 If -r is given, import statements will be rewritten to refer
 directly to the copied source code.
 
+if -tags="<BUILD TAGS>" is given, the dependencies will be generated
+with the specified build tags.
+
 For more about specifying packages, see 'go help packages'.
 `,
 	Run: runSave,
@@ -54,11 +57,13 @@ For more about specifying packages, see 'go help packages'.
 var (
 	saveCopy = true
 	saveR    = false
+	tags     = ""
 )
 
 func init() {
 	cmdSave.Flag.BoolVar(&saveCopy, "copy", true, "copy source code")
 	cmdSave.Flag.BoolVar(&saveR, "r", false, "rewrite import paths")
+	cmdSave.Flag.StringVar(&tags, "tags", "", "build tags")
 }
 
 func runSave(cmd *Command, args []string) {
@@ -96,7 +101,12 @@ func save(pkgs []string) error {
 	} else {
 		pkgs = []string{"."}
 	}
-	a, err := LoadPackages(pkgs...)
+	var a []*Package
+	if len(tags) > 0 {
+		a, err = LoadPackagesWithTags(tags, pkgs...)
+	} else {
+		a, err = LoadPackages(pkgs...)
+	}
 	if err != nil {
 		return err
 	}