Remove transitive loads

[apt-itude]

Fixes #54

This change creates explicit exports for all rules/macros in defs.bzl instead of relying on the deprecated behavior of implicitly exporting loaded symbols.

[CLAassistant]

All committers have signed the CLA.

[apt-itude]

Just a heads up: the buildifier check is failing because the latest version of the tool does not appear to have a -showlog flag. I ran it locally to make sure it's formatted properly. Maintainers, let me know if you want me to remove or change the flag to get CI passing.

[fwouts]

Thanks a lot!