Implemented simple 'bat pack' CLI command

No path rewriting yet, just simply copying the dependencies. They are
copied to the target directory + the path relative to the current working
directory. For example:

bat pack scenes/05-barber/05_02_C-agent_focused/05_02_C.lighting.blend /tmp/packer

will write to
/tmp/packer/scenes/05-barber/05_02_C-agent_focused/05_02_C.lighting.blend
and will copy ./path/to/file → /tmp/packer/path/to/file

blender_asset_tracer/cli/common.py

@@ -1,4 +1,5 @@
 """Common functionality for CLI parsers."""
+import pathlib
 
 
 def add_flag(argparser, flag_name: str, **kwargs):
@@ -14,3 +15,9 @@ def add_flag(argparser, flag_name: str, **kwargs):
                            **kwargs)
 
 
+def shorten(cwd: pathlib.Path, somepath: pathlib.Path) -> pathlib.Path:
+    """Return 'somepath' relative to CWD if possible."""
+    try:
+        return somepath.relative_to(cwd)
+    except ValueError:
+        return somepath