goose: add show command

Lists provider sets in packages given on the command line, including
outputs grouped by what is needed to obtain them.

The goose package now exports the loading phase as an API.

Example output: https://paste.googleplex.com/5509965720584192

Reviewed-by: Tuo Shan <shantuo@google.com>

internal/goose/goose.go

@@ -22,17 +22,7 @@ import (
 // Generate performs dependency injection for a single package,
 // returning the gofmt'd Go source code.
 func Generate(bctx *build.Context, wd string, pkg string) ([]byte, error) {
-	// TODO(light): allow errors
-	// TODO(light): stop errors from printing to stderr
-	conf := &loader.Config{
-		Build:               new(build.Context),
-		ParserMode:          parser.ParseComments,
-		Cwd:                 wd,
-		TypeCheckFuncBodies: func(string) bool { return false },
-	}
-	*conf.Build = *bctx
-	n := len(conf.Build.BuildTags)
-	conf.Build.BuildTags = append(conf.Build.BuildTags[:n:n], "gooseinject")
+	conf := newLoaderConfig(bctx, wd, true)
 	conf.Import(pkg)
 	prog, err := conf.Load()
 	if err != nil {
@@ -99,6 +89,24 @@ func Generate(bctx *build.Context, wd string, pkg string) ([]byte, error) {
 	return fmtSrc, nil
 }
 
+func newLoaderConfig(bctx *build.Context, wd string, inject bool) *loader.Config {
+	// TODO(light): allow errors
+	// TODO(light): stop errors from printing to stderr
+	conf := &loader.Config{
+		Build:               bctx,
+		ParserMode:          parser.ParseComments,
+		Cwd:                 wd,
+		TypeCheckFuncBodies: func(string) bool { return false },
+	}
+	if inject {
+		conf.Build = new(build.Context)
+		*conf.Build = *bctx
+		n := len(conf.Build.BuildTags)
+		conf.Build.BuildTags = append(conf.Build.BuildTags[:n:n], "gooseinject")
+	}
+	return conf
+}
+
 // gen is the generator state.
 type gen struct {
 	currPackage string