resolve regression in dependencies performance

[kevinushey]

Addresses #1999.

[kevinushey]

Some ad-hoc comparisons running in renv:

# renv 1.0.7
> system.time(renv::dependencies(quiet = TRUE))
   user  system elapsed 
  2.220   0.040   2.293 

# renv 1.0.8
> system.time(renv::dependencies(quiet = TRUE))
   user  system elapsed 
  8.161   0.172   6.968 

# dev
> system.time(renv::dependencies(quiet = TRUE))
   user  system elapsed 
  3.779   0.151   2.511