Need more solver heuristics

[gaborcsardi]

Cf. #96 (comment) and https://github.com/gaborcsardi/rgdal/runs/5552862022?check_suite_focus=true

• We should eliminate source packages if they are the same version.
• We should eliminate binary packages if they are the same version.

Let's see if anything else is needed.

[gaborcsardi]

A bad case with the learnr package:

pkgload::load_all()
library(pillar)

options(
  repos = c(
    RSPM = "https://packagemanager.rstudio.com/all/latest",
    CRAN = "https://cloud.r-project.org"
  )
)

prop <- new_pkg_installation_proposal(
  "deps::../learnr",
  config = list(dependencies = TRUE, library = tempfile())
)
prop$resolve()
prop$solve()