cmd/go: don't compute Embed fields if they're not needed

If the user provides the -json flag to explicitly specify fields, but
doesn't specify any *Embed* field, skip computing the embed fields.

This enhances the initial implementation of golang#29666.

src/cmd/go/internal/list/list.go

@@ -604,6 +604,11 @@ func runList(ctx context.Context, cmd *base.Command, args []string) {
 		// See issue #52443.
 		SuppressDeps:      !listJsonFields.needAny("Deps", "DepsErrors"),
 		SuppressBuildInfo: !listJsonFields.needAny("Stale", "StaleReason"),
+		SuppressEmbed: !listJsonFields.needAny(
+			"EmbedPatterns", "EmbedFiles",
+			"TestEmbedPatterns", "TestEmbedFiles",
+			"XTestEmbedPatterns", "XTestEmbedFiles",
+		),
 	}
 	pkgs := load.PackagesAndErrors(ctx, pkgOpts, args)
 	if !*listE {

src/cmd/go/internal/load/pkg.go

@@ -1919,12 +1919,14 @@ func (p *Package) load(ctx context.Context, opts PackageOpts, path string, stk *
 		p.Module = modload.PackageModuleInfo(ctx, pkgPath)
 	}
 
-	p.EmbedFiles, p.Internal.Embed, err = resolveEmbed(p.Dir, p.EmbedPatterns)
-	if err != nil {
-		p.Incomplete = true
-		setError(err)
-		embedErr := err.(*EmbedError)
-		p.Error.setPos(p.Internal.Build.EmbedPatternPos[embedErr.Pattern])
+	if !opts.SuppressEmbed {
+		p.EmbedFiles, p.Internal.Embed, err = resolveEmbed(p.Dir, p.EmbedPatterns)
+		if err != nil {
+			p.Incomplete = true
+			setError(err)
+			embedErr := err.(*EmbedError)
+			p.Error.setPos(p.Internal.Build.EmbedPatternPos[embedErr.Pattern])
+		}
 	}
 
 	// Check for case-insensitive collision of input files.
@@ -2774,6 +2776,9 @@ type PackageOpts struct {
 	// SuppressBuildInfo is true if the caller does not need p.Stale, p.StaleReason, or p.Internal.BuildInfo
 	// to be populated on the package.
 	SuppressBuildInfo bool
+
+	// SuppressEmbedFiles is true if the caller does not need any embedded files to be populated on the package.
+	SuppressEmbed bool
 }
 
 // PackagesAndErrors returns the packages named by the command line arguments