Fix crates with the same name as standard crates

The test and bin executables weren't getting the correct --extern flags when
tested and built, so the names were conflicting. This passes --extern for the
local crate to ensure the right crate is picked up.
auto merge of #247 : alexcrichton/cargo/extern-myself-please, r=carllerche

The test and bin executables weren't getting the correct --extern flags when
tested and built, so the names were conflicting. This passes --extern for the
local crate to ensure the right crate is picked up.