Extract FML command_line target (#26028)

Command-line argument parsing is valuable even in the absence of the
rest of FML's functionality.

This is pre-factoring for uwptool, which adds an adb-like helper tool
for install, uninstall, and launch of UWP applications for Windows
platforms.

See: https://github.com/flutter/flutter/issues/81756

fml/BUILD.gn

@@ -15,8 +15,6 @@ source_set("fml") {
     "base32.h",
     "build_config.h",
     "closure.h",
-    "command_line.cc",
-    "command_line.h",
     "compiler_specific.h",
     "concurrent_message_loop.cc",
     "concurrent_message_loop.h",
@@ -96,7 +94,7 @@ source_set("fml") {
     sources += [ "backtrace_stub.cc" ]
   }
 
-  public_deps = []
+  public_deps = [ ":command_line" ]
 
   deps = [
     "//third_party/dart/runtime:dart_api",
@@ -224,6 +222,18 @@ source_set("fml") {
   }
 }
 
+source_set("command_line") {
+  sources = [
+    "command_line.cc",
+    "command_line.h",
+  ]
+
+  public_configs = [
+    "//flutter:config",
+    "//flutter/common:flutter_config",
+  ]
+}
+
 if (enable_unittests) {
   test_fixtures("fml_fixtures") {
     fixtures = []