[CDRIVER-4705] Enable selective installation of libbson static/shared libraries (#1386)

* Delete unused CMake module for legacy packaging
* Use qualified paths for libbson #includes
* Add an additional indirection for common name mangling
* More compact config checks
* Lift common platform requirements to top-level config checks
* Define options to toggle build/install of dynamic/shared
* Add a mongo_target_requirements() convenience function
* Conditional build of libbson static/shared
* Install shared/static separately:

Each type can be disabled individually, and even packaged individually.
Installs two export sets that are lazily included based on their presence
at import-time.

* Remove usage of CMake export()

It is unlikely that this was used by any users, as we did not provide any
mechanism to import these, and the feature itself is somewhat obscure. It
complicated certain build arrangements due to its interactions with
BUILD_INTERFACE. It can be restored in the future if necessary.

* Dedup link options from pkg-config generation