nogo: Add a _default key to be a default config for all Analyzers.

This is useful for situations where you want to exclude/include the same directories for all the analyzers. Specifying a config for a particular analyzer will cause the _default config, if it exists, to be discarded for that analyzer (ie, the default config will not be merged).

You can use it like so:

{
	"_default": {
		"exclude_files": {
			"external/": "third_party",
			"proto/": "generated protobuf"
		}
	}
}

go/tools/builders/nogo_main.go

@@ -43,6 +43,8 @@ import (
 	"golang.org/x/tools/go/gcexportdata"
 )
 
+const nogoDefaultConfigName = "_default"
+
 func init() {
 	if err := analysis.Validate(analyzers); err != nil {
 		log.Fatal(err)
@@ -89,7 +91,7 @@ func run(args []string) error {
 		return fmt.Errorf("errors found by nogo during build-time code analysis:\n%s\n", diagnostics)
 	}
 	if *xPath != "" {
-		if err := ioutil.WriteFile(abs(*xPath), facts, 0666); err != nil {
+		if err := ioutil.WriteFile(abs(*xPath), facts, 0o666); err != nil {
 			return fmt.Errorf("error writing facts: %v", err)
 		}
 	}
@@ -380,6 +382,16 @@ func (g *goPackage) String() string {
 	return g.types.Path()
 }
 
+func mergeMap(a, b map[string]string) (out map[string]string) {
+	for k, v := range a {
+		out[k] = v
+	}
+	for k, v := range b {
+		out[k] = v
+	}
+	return out
+}
+
 // checkAnalysisResults checks the analysis diagnostics in the given actions
 // and returns a string containing all the diagnostics that should be printed
 // to the build log.
@@ -390,6 +402,10 @@ func checkAnalysisResults(actions []*action, pkg *goPackage) string {
 	}
 	var diagnostics []entry
 	var errs []error
+	var defaultConfig *config
+	if dc, ok := configs[nogoDefaultConfigName]; ok {
+		defaultConfig = &dc
+	}
 	for _, act := range actions {
 		if act.err != nil {
 			// Analyzer failed.
@@ -401,12 +417,17 @@ func checkAnalysisResults(actions []*action, pkg *goPackage) string {
 		}
 		config, ok := configs[act.a.Name]
 		if !ok {
-			// If the analyzer is not explicitly configured, it emits diagnostics for
-			// all files.
-			for _, diag := range act.diagnostics {
-				diagnostics = append(diagnostics, entry{Diagnostic: diag, Analyzer: act.a})
+			if defaultConfig != nil {
+				config = *defaultConfig
+				// Continue processing using the defaultConfig as config.
+			} else {
+				// If the analyzer is not explicitly configured, it emits diagnostics for
+				// all files.
+				for _, diag := range act.diagnostics {
+					diagnostics = append(diagnostics, entry{Diagnostic: diag, Analyzer: act.a})
+				}
+				continue
 			}
-			continue
 		}
 		// Discard diagnostics based on the analyzer configuration.
 		for _, d := range act.diagnostics {