I think this needs to be a CMake code formatting guideline.  It makes the code
more readable since CMake lacks commas to seprate arguments.

…nd_config_file() (TriBITSPub#299)

This is a reusable function that can be used with many other such
FindTPL<tplName>.cmake files that call find_package(<externalPkg>) that
defines imported targets.

…Pub#299)

Now all of the TribitsExampleProject2 TPLs Tpl1, Tpl2, Tpl3, and Tpl4 all can
call:

  tribits_external_package_create_imported_all_libs_target_and_config_file()

to be able to use the externally generated `<externalPkg>Config.cmake` files.

I also updated the tests for TribitsExampleProject2 and TribitsExampleApp2 to
ensure that find_package() is being called (by grepping the output).  I feel
pretty good about these tests.
TribitsExampleProject2

…SPub#299)

It just points to the maintainers guide for now.  May need a local users guide
reference for this.

…nabledTpl.cmake (TriBITSPub#299)

This makes it so that individual FindTPL<tplName>.cmake files don't need to
include this module.

I noticed this while working on TriBITSPub#299 so I decided to just fix this.

I did several things here:

* Updated howto for creating TriBITS FindTPL<tplName>.cmake files to cover all
  cases with more examples.  This also covers the new function
  tribits_external_package_create_imported_all_libs_target_and_config_file()

* Changed section name from "Addtional Topics" to "Miscellaneous Topics"

* Changes section name from "TriBITS TPL" to "TriBITS External Package/TPL"
  but provided anchor `TriBITS TPL`_ so I did not have to update all of the
  references.

* Provided subsections for TriBITS External Package/TPL files and variables

* Moved detailed material on tricky aspects of find_package() and
  <tplName>Config.cmake files to a new section "Tricky considerations for
  TriBITS-generated <tplName>Config.cmake files".

* Added references to documentation for the functions in the module
  TribitsExternalPackageWriteConfigFile.cmake.