Allow to have ArborX be a TPL

[Rombur]

With this PR, I can use an external ArborX with DTK. You need to set the following configuration variables:

-D DataTransferKit_ENABLE_ArborX_TPL=ON
-D TPL_ArborX_DIR=/path/to/arborx 

The PR is pretty straightforward, the only thing is that when ArborX is a TPL I create a dummy dtk_search library.

[Rombur]

Are all of them necessary?

They are necessary. Tribits is looking for these variables. At first, I had weird bugs where ArborX was a registered TPL at the beginning of the configuration (because one variable was set) but not at the end (because another one was not set).

[dalg24]

I expect only TPL_ENABLE_<TPL_NAME> and <TRIBITS_PACKAGE>_ENABLE_<TPL_NAME> are required