DPCLTDeviceMgr_GetDevices should not rely of the cached unordered map
of root sycl devices to sycl contexts. Because unordered map can change
the ordering.

Changed implementation to iterate over device::get_devices instead.

Confirmation of the fix:

```
In [1]: import dpctl

In [2]: d0 = dpctl.SyclDevice("gpu:0")

In [3]: d0.filter_string
Out[3]: 'opencl:gpu:0'

In [4]: [i for i, di in enumerate(dpctl.get_devices(dpctl.backend_type.all, dpctl.device_type.gpu)) if di == d0]
Out[4]: [0]

In [5]: d0 == dpctl.SyclDevice(d0.filter_string)
Out[5]: True

In [6]: d1 = dpctl.SyclDevice("gpu:1")

In [7]: d1.filter_string
Out[7]: 'level_zero:gpu:0'

In [8]: [i for i, di in enumerate(dpctl.get_devices(dpctl.backend_type.all, dpctl.device_type.gpu)) if di == d1]
Out[8]: [1]

In [9]: d1 == dpctl.SyclDevice(d1.filter_string)
Out[9]: True
```

… selector