build.cmds: Move build.App.cmd_xxx() here

For every cmd_xxx() method in build.App, create a class that's
instatiated for running the respective command. This has the
advantage of making App.py smaller (and faster), and having smaller,
more maintainable command modules adhering to a common interface.

Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/build/cmds/CmdLdflags.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+
+from argparse import Namespace, ArgumentParser
+
+from ..Cmd import Cmd
+
+class CmdLdflags(Cmd): # export
+
+    def __init__(self) -> None:
+        super().__init__('ldflags', help='ldflags')
+
+    def add_arguments(self, parser: ArgumentParser) -> None:
+        super().add_arguments(parser)
+        parser.add_argument('module', nargs='*', help='Modules')
+        parser.add_argument('--exclude', action='append', help='Exclude Modules', default=[])
+        parser.add_argument('-s', '--add-self', action='store_true',
+            default=False, help='Include libflags of specified modules, too, not only their dependencies')
+
+    def _run(self, args: Namespace) -> None:
+        print(self.app.get_ldflags(args.module, args.exclude, args.add_self))