Make package prefix requirement consistent

[pannous]

Currently wapm add needs a prefix, whereas wapm run cannot handle it:

wapm add mark/wasi-version   #OK
wapm add wasi-version # error
wapm run mark/wasi-version  #error
wapm run wasi-version  #OK

Please make package prefix requirement consistent, ideally make prefix optional in wapm add (use first / default) and allow it in wapm run (ignore it).