add -c flag for Starlark-in-java

**Related issues:**
- add -c flag [#35](bazelbuild/starlark#35)
- improve starlark-in-java installation documentation [#24](bazelbuild/starlark#24)

**Examples:**
$ bazel run :Starlark -- -c "print(1)"
1
$ bazel run :Starlark -- -c "print(1)" arg3
USAGE: Starlark [-c "\<cmdLineProgram\>" | \<fileName\>]

Closes #7683.

PiperOrigin-RevId: 239028987

src/main/java/com/google/devtools/starlark/Starlark.java

@@ -100,10 +100,20 @@ public void readEvalPrintLoop() {
  }
 
  /** Execute a Starlark file. */
- public int execute(String path) {
+ public int executeFile(String path) {
  String content;
  try {
  content = new String(Files.readAllBytes(Paths.get(path)), CHARSET);
+ return execute(content);
+ } catch (Exception e) {
+ e.printStackTrace();
+ return 1;
+ }
+ }
+
+ /** Execute a Starlark command. */
+ public int execute(String content) {
+ try {
  BuildFileAST.eval(env, content);
  return 0;
  } catch (EvalException e) {
@@ -119,10 +129,12 @@ public static void main(String[] args) {
  int ret = 0;
  if (args.length == 0) {
  new Starlark().readEvalPrintLoop();
- } else if (args.length == 1) {
- ret = new Starlark().execute(args[0]);
+ } else if (args.length == 1 && !args[0].equals("-c")) {
+ ret = new Starlark().executeFile(args[0]);
+ } else if (args.length == 2 && args[0].equals("-c")) {
+ ret = new Starlark().execute(args[1]);
  } else {
- System.err.println("too many arguments");
+ System.err.println("USAGE: Starlark [-c \"<cmdLineProgram>\" | <fileName>]");
  ret = 1;
  }
  System.exit(ret);