[cleanup] Limit number of hardcoded strings for layer type by leveraging QgsMapLayerFactory::typeToString()

[rouault]

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[rouault]

to knowleadgable devs, so we have a test failure

  File "/root/QGIS/tests/src/python/test_qgsprovidersublayerdetails.py", line 204, in test_to_mime
    self.assertEqual(uri.layerType, "pointcloud")
AssertionError: 'point-cloud' != 'pointcloud'

So it would seem that, in current master, QgsProviderSublayerDetails::toMimeUri() returns "pointcloud" whereas QgsMapLayerFactory::typeToSTring() returns "point-cloud"...
Which shows the need for uniformization sought by this PR, but I'm not sure what to do with that failure... Should we just adjust the failed test, or do we need to make https://github.com/qgis/QGIS/pull/60360/files#diff-a6c6864f28b601d260d5aff00f149039a0df622797f72f15311d052b1a38b77fL2482 accept "pointcloud" for backward compatibility ?