[sources.src] url=nosource for metapackages

[christophprokop]

At the moment we use dummy source for meta packages which shouldn't be necessary at all.
for example pkg.toml could contain a section like this:

[sources.src]
url=nosource
# hash.hash should not be needed then as well

[primeos-work]

@christophprokop I'd propose that we try to make specifying a source optional so that sources.src can be omitted entirely (seems like the cleanest solution) - would that be fine as well?

[christophprokop]

@primeos-work yes, that would help to clean up all the configurations that use dummy files at the moment