WIP Draft: Convert exemplar into an INTERFACE library

[ednolan]

This commit eliminates the src/ directory, moving
beman.examplar-config.cmake.in into the top level directory, moving the contents of src/beman/exemplar/CMakeLists.txt into the top level CMakeLists.txt, removing identity.cpp, and changing beman.exemplar from STATIC/SHARED into INTERFACE.

This commit is currently just intended to be a jumping-off point for discussion. What I would like to do if we are okay with this direction is to preserve the option for users to create STATIC/SHARED libraries by configuring cookiecutter differently. That feature is not present yet here.

Edit(River):
closes bemanproject/beman#175

Copy pasted from bemanproject/beman#175 :

Remaining todos:

• Update CI so it doesn't run a shared library build on the header only configuration
• Update the README to reflect an accurate install tree
• Update cookiecutter so that users can optionally configure the repository to create a src directory/static library/etc

[coveralls]

coverage: 100.0%. remained the same
when pulling 36dd20f on enolan_interface1
into ea33d50 on main.

[ednolan]

I opened a thread on Discourse here: https://discourse.bemanproject.org/t/converting-exemplar-to-a-cmake-interface-type-library/527

CC @bretbrownjr @nickelpro

[camio]

It's interesting how the header list moves to the top-level directory when it's a header-only library. This has two drawbacks:

1. A header-only -> with-source-file transition is more dramatic.
2. For a repository with multiple libraries, the top-level CMake file would grow quite large and we lose the "one CMakeLists.txt file per library" separation of concerns.

What are your thoughts on peeing the src/beman/exemplar directory and having it contain only a CMakeLists.txt that builds the target?

[steve-downey]

It shouldn't be necessary to have the list of files in the root, but I have found that creating the library and naming the header set at the root makes it more local.

https://github.com/steve-downey/optional/blob/main/CMakeLists.txt#L23-L28
and
https://github.com/steve-downey/optional/blob/main/include/beman/optional/CMakeLists.txt#L4-L15

[wusatosi]

Updated the TODOs. I might help out. Got the permission from @ednolan to push directly on this branch.