Remove the notion of "primary" from Context

Now that we have selective testing, this no longer makes any sense and all
queries to the path layout need to be based on the package being queried for.
This removes the primary flag from the Context, and requires that the `layout`
method have a local Package available

cc servo/servo#3580
auto merge of #675 : alexcrichton/cargo/fix-selective-test, r=brson

Now that we have selective testing, this no longer makes any sense and all
queries to the path layout need to be based on the package being queried for.
This removes the primary flag from the Context, and requires that the `layout`
method have a local Package available

cc servo/servo#3580