Untangle export files

ATTENTION: This pull request is tied to [Trilinos bug  #6228](https://software.sandia.gov/bugzilla/show_bug.cgi?id=6228) which must be resolved first.

This pull request is concerned with the structure of the export files.

Up until now, all package export files `include`d the main project configuration export file to get access to their dependent packages. Also, there was only one target export file that incorporated all targets built by the project.

The present pull request changes this:
 * Every package export file now has a accompanying target export file.
 * All packages include the configuration files of their dependencies, and nothing more.

Everything that worked previously still works, in particular the project export files contain the same information as they did before.

One exception is that the pull requires CMake's deprecated `SUBDIRS()` command not to be used. The reason for this is that we need to be sure now that `ADD_LIBRARY()` is called *before* the `EXPORT` files are installed. `SUBDIRS()` works unpredictably (or probably rather in unexpected ways).