GenerateID() produces unusable name for type in global namespace

### The problem

When writing Python configs for a component, if an ID is required for a type within the global namespace (i.e. a C struct) the auto-generated name is simply the name of the type, which is unusable as a name. E.g.

```
lv_obj_t = cg.global_ns.struct("lv_obj_t")

OBJ_SCHEMA = cv.Schema(
{
cv.GenerateID(): cv.declare_id(lv_obj_t),
}
)
```

If no id is provided in the yaml, the auto-generated ID name is `lv_obj_t` which is a type name.

The problem is inside the core.ID class:

```
def resolve(self, registered_ids):
from esphome.config_validation import RESERVED_IDS

if self.id is None:
base = str(self.type).replace("::", "_").lower()
name = "".join(c for c in base if c.isalnum() or c == "_")
used = set(registered_ids) | set(RESERVED_IDS) | CORE.loaded_integrations
self.id = ensure_unique_string(name, used)
return self.id

```
So where no name is provided, the base name is derived by replacing `::` with `_`, but if there is no namespace in the type name, this just returns the undecorated type.

Possible fix (working for me):

```
if self.id is None:
base = str(self.type).replace("::", "_").lower()
if base == str(self.type):
base = "_ns_" + str(self.type)
name = "".join(c for c in base if c.isalnum() or c == "_")
used = set(registered_ids) | set(RESERVED_IDS) | CORE.loaded_integrations
self.id = ensure_unique_string(name, used)
return self.id

```

### Which version of ESPHome has the issue?

dev

### What type of installation are you using?

Home Assistant Add-on

### Which version of Home Assistant has the issue?

_No response_

### What platform are you using?

ESP8266

### Board

_No response_

### Component causing the issue

_No response_

### Example YAML snippet

_No response_

### Anything in the logs that might be useful for us?

_No response_

### Additional information

_No response_