lintcmd: allow use from library, without forcing a call to os.Exit

Split Command.Run into Command.Execute and Command.Run, where the former
is responsible for running analyzers and reporting their results and the
latter for calling os.Exit with the former's result. This allows running
the Staticcheck CLI programmatically together with other tools.

Closes: gh-1597 [via git-merge-pr]

lintcmd/cmd.go

@@ -271,9 +271,9 @@ func decodeGob(br io.ByteReader) ([]run, error) {
 	return runs, nil
 }
 
-// Run runs all registered analyzers and reports their findings.
-// It always calls os.Exit and does not return.
-func (cmd *Command) Run() {
+// Execute runs all registered analyzers and reports their findings.
+// The status code returned can be used for os.Exit(cmd.Execute()).
+func (cmd *Command) Execute() int {
 	// Set up profiling and tracing
 	if path := cmd.flags.debugCpuprofile; path != "" {
 		f, err := os.Create(path)
@@ -332,8 +332,13 @@ func (cmd *Command) Run() {
 		trace.Stop()
 	}
 
-	// Exit with appropriate status
-	os.Exit(exit)
+	return exit
+}
+
+// Run runs all registered analyzers and reports their findings.
+// It always calls os.Exit and does not return.
+func (cmd *Command) Run() {
+	os.Exit(cmd.Execute())
 }
 
 func (cmd *Command) analyzersAsSlice() []*lint.Analyzer {