Disallow nesting of packages with same packge_id
Disallow nesting of packages with same packge_id

If a package contains a subdirectory that contains a copy of
itself (strange, but encountered in the wild in the context of an OS
package build tool), cargo was failing with a non-descriptive "An
unknown error occurred" (or "no package found in source" with
--verbose).

Since nested packages use the root path, such a package will have an
identical package_id to the root package. With this patch, we keep the
first occurance of a given package_id rather than overwritting --
effectively ignoring the nested package with duplicated name. A warn!
message is printed.

brson/cargo,JacobHenner/cargo,aturon/cargo,tshepang/cargo,iliekturtles/cargo,Gankro/cargo,apjanke/cargo,worr/cargo,tafia/cargo,Ticki/cargo,felixbuenemann/cargo,JacobHenner/cargo,huonw/cargo,youprofit/cargo,mceier/cargo,iliekturtles/cargo,Kimundi/cargo,pcwalton/cargo,liigo/cargo,Kimundi/cargo,winger/cargo,iliekturtles/cargo,cmr/cargo,liigo/cargo,JacobHenner/cargo,durka/cargo,tsurai/cargo,alex/cargo,brson/cargo,Kimundi/cargo,Frederick888/cargo,pcwalton/cargo,cmr/cargo,vojtechkral/cargo,blaenk/cargo,rustyhorde/cargo,mceier/cargo,frewsxcv/cargo,sbeckeriv/cargo,akiss77/cargo,gchp/cargo,eminence/cargo,nipunn1313/cargo,youprofit/cargo,AndiDog/cargo,alex/cargo,eminence/cargo,mceier/cargo,AndiDog/cargo,vojtechkral/cargo,durka/cargo,ThinTim/cargo,beni55/cargo,blaenk/cargo,huonw/cargo,tsurai/cargo,vi/cargo,winger/cargo,sbeckeriv/cargo,mceier/cargo,nipunn1313/cargo,brson/cargo,AndiDog/cargo,ThinTim/cargo,Gankro/cargo,durka/cargo,durka/cargo,ArtemGr/cargo,vi/cargo,mark-buer/cargo,liigo/cargo,beni55/cargo,youprofit/cargo,tsurai/cargo,akiss77/cargo,winger/cargo,alex/cargo,tafia/cargo,worr/cargo,huonw/cargo,blaenk/cargo,felixbuenemann/cargo,cmr/cargo,Turbo87/cargo,tshepang/cargo,tafia/cargo,gchp/cargo,fhahn/cargo,achanda/cargo,gchp/cargo,mark-buer/cargo,huonw/cargo,ucarion/cargo,felixbuenemann/cargo,JacobHenner/cargo,youprofit/cargo,blaenk/cargo,tshepang/cargo,blaenk/cargo,brson/cargo,aturon/cargo,apjanke/cargo,Kimundi/cargo,rust-lang/cargo,JacobHenner/cargo,ArtemGr/cargo,Frederick888/cargo,akiss77/cargo,ThinTim/cargo,rustyhorde/cargo,apjanke/cargo,Frederick888/cargo,ArtemGr/cargo,Frederick888/cargo,eminence/cargo,mceier/cargo,shaleh/cargo,vyskocilm/cargo,liigo/cargo,beni55/cargo,Gankro/cargo,shaleh/cargo,brson/cargo,worr/cargo,mark-buer/cargo,shaleh/cargo,tafia/cargo,rustyhorde/cargo,sbeckeriv/cargo,vojtechkral/cargo,tafia/cargo,apjanke/cargo,cesarb/cargo,rust-lang/cargo,winger/cargo,vyskocilm/cargo,tshepang/cargo,cesarb/cargo,vojtechkral/cargo,fhahn/cargo,pcwalton/cargo,iliekturtles/cargo,Frederick888/cargo,vi/cargo,apjanke/cargo,sbeckeriv/cargo,Gankro/cargo,tsurai/cargo,Gankro/cargo,winger/cargo,cmr/cargo,vyskocilm/cargo,Ticki/cargo,Ticki/cargo,AndiDog/cargo,ucarion/cargo,ThinTim/cargo,vyskocilm/cargo,akiss77/cargo,ThinTim/cargo,felixbuenemann/cargo,ArtemGr/cargo,tsurai/cargo,frewsxcv/cargo,huonw/cargo,aturon/cargo,fhahn/cargo,fhahn/cargo,frewsxcv/cargo,Ticki/cargo,Kimundi/cargo,youprofit/cargo,iliekturtles/cargo,akiss77/cargo,achanda/cargo,gchp/cargo,Ticki/cargo,achanda/cargo,vi/cargo,frewsxcv/cargo,Turbo87/cargo,nipunn1313/cargo,worr/cargo,liigo/cargo,worr/cargo,frewsxcv/cargo,rustyhorde/cargo,beni55/cargo,rust-lang/cargo,achanda/cargo,vyskocilm/cargo,gchp/cargo,nipunn1313/cargo,cmr/cargo,mark-buer/cargo,rust-lang/cargo,pcwalton/cargo,ucarion/cargo,pcwalton/cargo,cesarb/cargo,vi/cargo,Turbo87/cargo,Turbo87/cargo,aturon/cargo,AndiDog/cargo,shaleh/cargo,mark-buer/cargo,eminence/cargo,ucarion/cargo,alex/cargo,aturon/cargo,shaleh/cargo,ArtemGr/cargo,cesarb/cargo,felixbuenemann/cargo,ucarion/cargo,beni55/cargo,alex/cargo,Turbo87/cargo,cesarb/cargo,fhahn/cargo