Add renv and packrat to default-excluded directories

[russHyde]

#492
Suggest modifying the exclusions param in lint_dir and linr_package to drop common directories (renv, packrat)

[AshesITR]

That usecase was the main motivation for me to code up the directory exclusion as well. The risk for unintended exclusion of those two is minimal so making them the default sounds like a good idea.

[AshesITR]

NB why would one use renv or packrat in a Package?
Maybe better to only add this default for lint_dir. WDYT?

[russHyde]

Sorry, you are right. I never use either.