Rename use_tidy_versions()

[jennybc]

use_tidy_versions() is not parallel to use_tidy_ci() or use_tidy_github(), i.e. it's not enforcing a policy we embrace unconditionally. Aggressive minimum versions are another feature that can make a package feel like a heavy dependency (in addition to having lots of dependencies).

[jennybc]

This should possibly get split into 2 functions, instead of 1 function with source "local" vs source = "CRAN".

Brainstorming names:

• use_installed_versions()
• use_local_versions()
• use_cran_versions()
• use_released_versions()

[hadley]

I gave relatively little thought to the name because I don't think it's something anyone should be doing routinely. I added @keyword internal to hopefully reduce the number of people who stumble across it accidentally.