Whenever unused definitions are removed, some new definitions may appear
as unused. This PR goes does this rabbit hole to delete all unused
definitions until no unused candidate shows up.

* contributes go-swagger/go-swagger#2657

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>

* fix(ci): muted warnings in CI runs due to cache conflicts
* ci: fixed test coverage for sub packages
* tests: added unit tests to the internal packages

This recovers a bit of the loss of test coverage ratio after accounting
for all source files.

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>

This reproduces the $ref issue with relative paths in sibling folders
in responses (same applies to parameters, but not schemas).

Added a test to verify that:
* (i) the invalid path that used to work no longer does
* (ii) the valid path that used not to work now does

* contributes go-swagger/go-swagger#2743

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>