Don't require test dependencies when building

Right now it seems like if a PPX test mentions a module, the dependency providing that module must be installed when building the tested code.
This should be made unnecessary, so that only building with runtest requires extra dependencies.

No files or tests are named. Start by reproducing a build where a PPX test references a module supplied by an extra dependency, then inspect how normal builds and runtest handle those dependencies. Done means the tested code builds without the test dependency, while runtest still requires it.

Written by the indexing model from the issue text.