drake is excessively hashing imported files

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.
• Advanced users: verify that the bug still persists in the current development version (i.e. remotes::install_github("ropensci/drake")) and mention the SHA-1 hash of the Git commit you install.

Description

See #869 (comment) for a reprex. The last 2 make()s should be practically instantaneous. Instead, they take 5 seconds. Related: #869.

[wlandau]

I thought I took care of this way back in #4, but apparently a bug came back that caused files to be rehashed too often. Fixed in 3a696ea.