Resolve Module Proxies

[t-kalinowski]

This is a followup to #1552 (and to a smaller extent, #1558 and #1560).

This standardizes the approach across py_{has,get,set,del}_{attr,item} to consistently initialize Python and resolve module proxies as needed.

The following functions will resolve delayed module proxies (and initialize Python). These are unchanged from the previous CRAN release:

• py_get_item()
• py_set_item()
• py_del_item()
• py_has_attr()
• py_get_attr()
• py_set_attr()
• py_del_attr()
• py_list_attributes()

With this patch, py_validate_xptr() will now also resolve module proxies (previously it would error).

The following functions do not resolve module proxies or initialize Python:

• py_is_null_xptr() (returns TRUE for module proxies)
• print() and str() (printing a lazy module does not resolve it or initialize python).

This also fixes two additional minor issue:

• printing delayed module now outputs the same output as printing a loaded module, instead of <pointer: 0x0>.
• py_get_item(silent = TRUE) now clears any set exceptions, instead of restoring the previous exception state (to avoid a repeat of Call PyErr_Clear() instead of PyErr_Restore() in py_get_attr_impl(silent = TRUE) #1560)