[api] further speed improvements in webui#project_infos:

In Project#expand_all_packages don't bother loading all packages as
ruby objects if all we want to know is package name and project name

reduces runtime from ~590ms to ~370ms (the runtime varies a lot due
to backend parts involved, but I ran before and after each 4 times)

Additionally /source/<prj> also gets a gain

comparing /source/openSUSE:Factory and /source/openSUSE:Factory:Staging
with and without expand:

before
 O:F 727ms  O:F?expand=1 350ms
 O:F:S 9ms  O:F:S?expand=1 8722ms

after
 O:F 87ms OF?expand=1 111ms
 O:F:S 11ms O:F:S?expand=1 79ms

So factor 110 :)

Point of data: /source/openSUSE:Factory from srcsrv takes 172ms here,
/source/openSUSE:Factory:Staging?expand=1 take 240ms