Auto merge of #5231 - matklad:simplify, r=alexcrichton
Auto merge of #5231 - matklad:simplify, r=alexcrichton

Simplify `build_map`

This doesn't change semantics, but helps us not to rebuild BuildScripts
twice, and makes sure that we fail with assert if somehow we get cyclic
deps.

r? @alexcrichton

tsurai/cargo,tshepang/cargo,rustyhorde/cargo,rustyhorde/cargo,vojtechkral/cargo,tshepang/cargo,rustyhorde/cargo,tshepang/cargo,nipunn1313/cargo,tsurai/cargo,rust-lang/cargo,nipunn1313/cargo,tsurai/cargo,rust-lang/cargo,tsurai/cargo,nipunn1313/cargo,tshepang/cargo,eminence/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo,vojtechkral/cargo,eminence/cargo,vojtechkral/cargo,tsurai/cargo,rustyhorde/cargo,rust-lang/cargo,eminence/cargo,vojtechkral/cargo