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digitalmars.D.learn - what is local package map(local-packages.json) for dub while doing

$ 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?
Jan 31