Move filename/stdin parsing logic to ParsedArgs

Checking whether we have a single filename '-' or multiple files, none
of which are '-' is parsing and should be part of the parsing step

core/src/main/java/com/facebook/ktfmt/cli/Main.kt

@@ -56,10 +56,6 @@ class Main(
       }
       val result = mutableListOf<File>()
       for (arg in args) {
-        if (arg == "-") {
-          error(
-              "Error: '-', which causes ktfmt to read from stdin, should not be mixed with file name")
-        }
         result.addAll(
             File(arg).walkTopDown().filter {
               it.isFile && (it.extension == "kt" || it.extension == "kts")
@@ -95,14 +91,8 @@ class Main(
       return 1
     }
 
-    val files: List<File>
-    try {
-      files = expandArgsToFileNames(parsedArgs.fileNames)
-    } catch (e: java.lang.IllegalStateException) {
-      err.println(e.message)
-      return 1
-    }
-
+    val files: List<File> = expandArgsToFileNames(parsedArgs.fileNames)
+
     if (files.isEmpty()) {
       err.println("Error: no .kt files found")
       return 1

core/src/main/java/com/facebook/ktfmt/cli/ParsedArgs.kt

@@ -76,6 +76,14 @@ data class ParsedArgs(
         }
       }
 
+      if (fileNames.contains("-") && fileNames.size > 1) {
+        val filesExceptStdin = fileNames - "-"
+        return ParseResult.Error(
+          "Cannot read from stdin and files in same run. Found stdin specifier '-'" +
+              " and files ${filesExceptStdin.joinToString(", ")} "
+        )
+      }
+
       return ParseResult.Ok(
           ParsedArgs(
               fileNames,

core/src/test/java/com/facebook/ktfmt/cli/MainTest.kt

@@ -104,16 +104,6 @@ class MainTest {
         .containsExactly(foo1, bar1, foo2, bar2)
   }
 
-  @Test
-  fun `expandArgsToFileNames - a dash is an error`() {
-    try {
-      Main.expandArgsToFileNames(listOf(root.resolve("foo.bar").toString(), File("-").toString()))
-      fail("expected exception, but nothing was thrown")
-    } catch (e: IllegalStateException) {
-      assertThat(e.message).contains("Error")
-    }
-  }
-
   @Test
   fun `Using '-' as the filename formats an InputStream`() {
     val code = "fun    f1 (  ) :    Int =    0"

core/src/test/java/com/facebook/ktfmt/cli/ParsedArgsTest.kt

@@ -121,6 +121,12 @@ class ParsedArgsTest {
     assertThat(parseResult).isInstanceOf(ParseResult.Error::class.java)
   }
 
+  @Test
+  fun `parseOptions rejects '-' and files at the same time`() {
+    val parseResult = ParsedArgs.parseOptions(arrayOf("-", "File.kt"))
+    assertThat(parseResult).isInstanceOf(ParseResult.Error::class.java)
+  }
+
   @Test
   fun `processArgs use the @file option with non existing file`() {
     val e =