Improve BLT's export logic

We should help with exporting CMake targets better. The following two ideas were proposed:

1. Add export options to `blt_add_library`, for example, export namespace and export set name. This could probably be just one optional parameter: `blt_add_library(NAME foo ... EXPORT myProjectName)`
2. Improve `blt_export_tpl_targets()` in two ways: first, add the ability to export project specific TPLs under your projects namespace, and second, have blt export its targets to a blt specific namespace and have the exported targets be granular so that you can add targets as you build up your connected projects (think, camp exporting from raja, then umpire adding mpi because neither needed them)

Start by locating the definitions of blt_add_library and blt_export_tpl_targets, then trace the existing CMake target export logic and related tests. Compare the current behavior with the two proposed export approaches in the issue. Done means the supported export options and namespace behavior are specified and covered for project targets, project-specific TPLs, and granular BLT targets.

Written by the indexing model from the issue text.