deps/npm/lib/build.js
function dependentLink (pkg, cb) {
  asyncMap(pkg._resolvedDeps, function (dep, cb) {
    var dependents = path.join(npm.dir, dep.name, dep.version, "dependents")
      , to = path.join(dependents, pkg.name + "@" + pkg.version)
      , from = path.join(npm.dir, pkg.name, pkg.version)
    link(from, to, cb)
  }, cb)
}
function dependentLink (pkg, cb) {
  asyncMap(pkg._resolvedDeps, function (dep, cb) {
    var dependents = path.join(npm.dir, dep.name, dep.version, "dependents")
      , to = path.join(dependents, pkg.name + "@" + pkg.version)
      , from = path.join(npm.dir, pkg.name, pkg.version)
    link(from, to, cb)
  }, cb)
}
function dependentLink ( pkg , cb ) { asyncMap ( pkg . _resolvedDeps , function ( dep , cb ) { var dependents = path . join ( npm . dir , dep . name , dep . version , "dependents" ) , to = path . join ( dependents , pkg . name + "@" + pkg . version ) , from = path . join ( npm . dir , pkg . name , pkg . version ) link ( from , to , cb ) } , cb ) }
for each dependency, link this pkg into the proper "dependent" folder
for each dependency link this pkg into the proper dependent folder