Error message in case of circular dependencies

[nbenn]

This is a duplicate of #357 (with a different scope for a possible "fix"):

Having two packages (A and B) directly depend on each other, as part of a call to pak::lockfile_install(), will generate an error as

Error: 
! error in pak subprocess
Caused by error in `select_next_task(state)`:
! Cannot select new package installation task.
ℹ 2 packages still waiting to install: A and B.
ℹ This is an internal error in pkgdepends, please report an issue at
  <https://github.com/r-lib/pkgdepends/issues>.

Ofc this is a "user error". Nevertheless: a more specific error msg could very much help getting to the underlying issue quicker.

The same issue also came up in #306, where a more telling error message could have been helpful as well.

(I am opening this here even though the problem came up with pak, on account of clear directions to do so in the mentioned error msg.)