watched nested dependencies throws AttributeError

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#### ALL software version info
(this library, plus any other relevant software, e.g. bokeh, python, notebook, OS, browser, etc)
param v 1.12.3
Reproducible at least on on python 3.10 on Ubuntu.
#### Description of expected behavior and the observed behavior
The error below is thrown when I try nested parameterized dependencies, where there is a watch on a nested dependency. I would expect that the watch flag should work in these circumstances.

#### Complete, minimal, self-contained example code that reproduces the issue

```
import param

class A(param.Parameterized):
x=param.Number(default=0)

class B(param.Parameterized):
a=param.ClassSelector(A, default=A())

@param.depends('a.x', watch=True)
def y(self):
pass

class C(param.Parameterized):
b=param.ClassSelector(B, default=B())

c = C()

```

#### Stack traceback and/or browser JavaScript console output
```traceback (most recent call last):
File "/home/mats/miniconda3/envs/quant/lib/python3.10/site-packages/IPython/core/interactiveshell.py", line 3398, in run_code
exec(code_obj, self.user_global_ns, self.user_ns)
File "", line 1, in
runfile('/home/mats/.config/JetBrains/PyCharm2022.2/scratches/scratch_2.py', wdir='/home/mats/.config/JetBrains/PyCharm2022.2/scratches')
File "/home/mats/.local/share/JetBrains/Toolbox/apps/PyCharm-P/ch-0/222.4345.23/plugins/python/helpers/pydev/_pydev_bundle/pydev_umd.py", line 198, in runfile
pydev_imports.execfile(filename, global_vars, local_vars) # execute the script
File "/home/mats/.local/share/JetBrains/Toolbox/apps/PyCharm-P/ch-0/222.4345.23/plugins/python/helpers/pydev/_pydev_imps/_pydev_execfile.py", line 18, in execfile
exec(compile(contents+"\n", file, 'exec'), glob, loc)
File "/home/mats/.config/JetBrains/PyCharm2022.2/scratches/scratch_2.py", line 20, in
c = C()
File "/home/mats/miniconda3/envs/quant/lib/python3.10/site-packages/param/parameterized.py", line 3173, in __init__
self.param._setup_params(**params)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/site-packages/param/parameterized.py", line 1387, in override_initialization
fn(parameterized_instance, *args, **kw)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/site-packages/param/parameterized.py", line 1633, in _setup_params
self.param._instantiate_param(p)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/site-packages/param/parameterized.py", line 1688, in _instantiate_param
new_object = copy.deepcopy(param_obj.default)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 172, in deepcopy
y = _reconstruct(x, memo, *rv)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 271, in _reconstruct
state = deepcopy(state, memo)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 146, in deepcopy
y = copier(x, memo)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 231, in _deepcopy_dict
y[deepcopy(key, memo)] = deepcopy(value, memo)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 172, in deepcopy
y = _reconstruct(x, memo, *rv)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 297, in _reconstruct
value = deepcopy(value, memo)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 146, in deepcopy
y = copier(x, memo)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 206, in _deepcopy_list
append(deepcopy(a, memo))
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 172, in deepcopy
y = _reconstruct(x, memo, *rv)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 265, in _reconstruct
y = func(*args)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 264, in
args = (deepcopy(arg, memo) for arg in args)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 172, in deepcopy
y = _reconstruct(x, memo, *rv)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/copy.py", line 273, in _reconstruct
y.__setstate__(state)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/site-packages/param/parameterized.py", line 3227, in __setstate__
watcher_args[2] = _m_caller(self, fn._watcher_name)
File "/home/mats/miniconda3/envs/quant/lib/python3.10/site-packages/param/parameterized.py", line 661, in _m_caller
function = getattr(self, method_name)
AttributeError: 'A' object has no attribute 'y'
```

When a default value is given to a ClassSelector (like in C above), a deepcopy of that default value happens, and it is the deepcopy that is causing the exception, so a shorter example wold be to just do:
```
import param
import copy

class A(param.Parameterized):
x=param.Number(default=0)

class B(param.Parameterized):
a=param.ClassSelector(A, default=A())

@param.depends('a.x', watch=True)
def y(self):
pass

copy.deepcopy(B())

```

Removing the "watch" flag removes the issue, but I also lose the watched updates that I need.