Restructure SeedingLayerSetsBuilder

- Move edm::ParameterSet parsing to SeedingLayerSetsBuilder::LayerSpec
  constructor from SeedingLayerSetsBuilder constructor. This makes the
  roles clearer, and allows couple of LayerSpec members to be const.
- Add DetLayer pointers as members of SeedingLayerSetsBuilder, in
  order to have access to them also outside
  SeedingLayerSetsBuilder::layers() (to be committed later).
- Store LayerSpec's as one LayerSpec per unique Layer, and use indices
  (theLayerSetIndices) to build the layer sets, similarly to
  SeedingLayerSetsHits.
- Store layer names and DetLayer pointers in separate vectors. Layer
  names are configuration-time constants, and SeedingLayerSetsHits
  would like to have a pointer to the vector (which has only the
  names). DetLayer pointers are updated in updateEventSetup() method,
  as often as the TrackerRecoGeometryRecord is updated. Having them in
  a separate vector makes it easier to pass them to
  SeedingLayerSetsHits, and keeps LayerSpec objects untouched after
  their construction.

With these changes, SeedingLayerSets hits can store just pointers to
theLayerNames and theLayerSetIndices, both of which are
configuration-time constants.
Change LogError to cms::Exception

This is anyway rather catastrophic failure from mismatch between
SeedingLayer configuration and conditions.
Add theTTRHBuilders member to SeedingLayerSetsBuilder

To allow obtaining hits directly from HitExtractor, bypassing
SeedingLayer (to be committed later).
Remove hasPredefinedHitErrors from SeedingLayer(SetsBuilder)

It is not used anywhere, and it makes the code simpler to remove it.
Deliver TTRHBuilder to HitExtractor instead of SeedingLayer itself

Only the TTRHBuilder was used from SeedingLayer anyway.
Add SeedingLayersEDProducer-friendly interface to SeedingLayerSetsBuilder
Migrate SeedingLayerSetsHits and SeedingLayersEDProducer to the new interface of SeedingLayerSetsBuilder
Rewrite SeedingLayerSetsBuilder::hits()
Deliver hits to SeedingLayerSetsHits with swap-style interface