Resolve non watched references too to catch bad references

[maximlt]

Fixes #357

@philippjfr I'm very far from being confident with this change.

This example:

import param

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

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

Would immediately fail at the class creating time with:

Traceback (most recent call last):
  File ".mltmess/issue_params.py", line 47, in <module>
    class A(param.Parameterized):
  File "/Users/mliquet/dev/param/param/parameterized.py", line 2903, in __init__
    deps, dynamic_deps = _params_depended_on(minfo, dynamic=False)
  File "/Users/mliquet/dev/param/param/parameterized.py", line 640, in _params_depended_on
    ddeps, ddynamic_deps = (minfo.cls if minfo.inst is None else minfo.inst).param._spec_to_obj(d, dynamic, intermediate)
  File "/Users/mliquet/dev/param/param/parameterized.py", line 2592, in _spec_to_obj
    raise AttributeError(f"Attribute {attr!r} could not be resolved on {src}.")
AttributeError: Attribute 'y' could not be resolved on <class '__main__.A'>.

[philippjfr]

My guess is that this breaks cases like this, which we didn't intend to support but did accidentally?

class A(param.Parameterized):

    def __init__(self):
        self.x = pn.widgets.FloatSlider()
        super().__init__()

    @param.depends('x.value')
    def update_on_x(self):
        return self.x.value * 2

[philippjfr]

Nvm, still seems to work. Seems okay.

[maximlt]

I added a test based on the comment above and ran the unit test suites of Panel and HoloViews with that branch checked out and saw no problem, I'm going to merge.