As Clara is no longer maintained as a separate project, the
implementation was moved to the internal subfolder of top-level
folder. This removes one folder and avoids potential user confusion.

Also simplified the convenience header checking script accordingly.

There are two reasons for this:

1) It is highly unlikely that someone has use for this header,
which has no customization points and only provides simplest
possible main, and cannot link the static library which also
provides a default main implementation.
2) It being a header was causing extra complications with
the convenience headers, and our checking script. This would either
require special handling in the checking script, or would break user's
of the main convenience header.

All in all, it is simpler and better in the long term to remove it,
than to fix its problems.

Development build enables warnings and and `Werror` or equivalent.

Previously, we would collect coverage data for all source files in
Catch2's directory, including tests and examples, and we would then
ask codecov.io to ignore those. With this change, OpenCppCoverage
only collects coverage data for source files in the `src/` directory.
This cuts the size of the coverage report in half, and also speeds
up the coverage collection.