Fixes #447

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
```

Author: oleksandr-pavlyk

dpctl-capi/source/dpctl_sycl_device_manager.cpp

@@ -146,15 +146,18 @@ DPCTLDeviceMgr_GetDevices(int device_identifier)
     } catch (std::bad_alloc const &ba) {
         return nullptr;
     }
-    auto &cache = DeviceCacheBuilder::getDeviceCache();
+    const auto &root_devices = device::get_devices();
+    default_selector mRanker;
 
-    for (const auto &entry : cache) {
+    for (const auto &root_device : root_devices) {
+        if (mRanker(root_device) < 0)
+            continue;
         auto Bty(DPCTL_SyclBackendToDPCTLBackendType(
-            entry.first.get_platform().get_backend()));
+            root_device.get_platform().get_backend()));
         auto Dty(DPCTL_SyclDeviceTypeToDPCTLDeviceType(
-            entry.first.get_info<info::device::device_type>()));
+            root_device.get_info<info::device::device_type>()));
         if ((device_identifier & Bty) && (device_identifier & Dty)) {
-            Devices->emplace_back(wrap(new device(entry.first)));
+            Devices->emplace_back(wrap(new device(root_device)));
         }
     }
     // the wrap function is defined inside dpctl_vector_templ.cpp