Internal helper used by check_project(). Returns TRUE only when the
lockfile parses and records no packages other than renv itself.
Details
renv is discounted because renv::scaffold() installs it into the
project library and records it, so the lockfile of a freshly scaffolded
project is not literally empty. What matters is whether anything the
analysis depends on is in there.
An unparseable lockfile returns FALSE: that is a different problem and
this helper should not report it as this one.
