$ dub build -b release -v


Using dub registry url 'https://code.dlang.org/'
Refreshing local packages (refresh existing: true)...
Looking for local package map at /var/lib/dub/packages/local-packages.json
Looking for local package map at /home/john/.dub/packages/local-packages.json
Refreshing local packages (refresh existing: false)...
Looking for local package map at /var/lib/dub/packages/local-packages.json
Looking for local package map at /home/john/.dub/packages/local-packages.json
  Found dependency gtk-d 3.7.3
    Found dependency gtk-d:sv 3.7.3
      Found dependency gtk-d:gtkd 3.7.3
    Found dependency gtk-d:gstreamer 3.7.3
    Found dependency gtk-d:peas 3.7.3
    Found dependency gtk-d:vte 3.7.3
Refreshing local packages (refresh existing: false)...
Looking for local package map at /var/lib/dub/packages/local-packages.json
Looking for local package map at /home/john/.dub/packages/local-packages.json
  Found dependency gtk-d 3.7.3
    Found dependency gtk-d:sv 3.7.3
      Found dependency gtk-d:gtkd 3.7.3
    Found dependency gtk-d:gstreamer 3.7.3
    Found dependency gtk-d:peas 3.7.3
    Found dependency gtk-d:vte 3.7.3
Checking for upgrades.
Using cached upgrade results...
Generating using build


The verbose message is confusing. Why are there so many attempts to look for local package map? And I don't even have it. How can I generate one if it's useful?

What does "Checking for upgrades" do?