Fixed the bogus dev-dependencies pull in on regular build

[farcaller]

Do not pull in dev-dependencies unless it's a test or bench build (see #1796 for additional details).

The primary use case is proper cross-building of crates with unit tests, where it's impossible (or highly unreasonable) to cross-build the test frameworks.

[rust-highfive]

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[alexcrichton]

Thanks! Could you also add a test for this?

[farcaller]

This patch breaks test_cargo_test::example_dev_dep.

My own opinion is that the example code shouldn't pull in the dev deps, as having example binaries being cross-compiled is perfectly valid use case while I can hardly imagine the need to use any additional code in examples that are not covered by main crate deps. Any advice on why that test exists in a way it exists?

[alexcrichton]

That's a test for how examples are specc'd which is that examples have access to dev-dependencies (e.g. it's a legit failure)

[farcaller]

After discussing over irc I now understand better how exactly it is legit. I will try to dig deeper to make a fix for original issue in #1796 without breaking the example use case.

[farcaller]

This seem to be a much bigger issue then two LoC change, I'm closing this PR continuing my investigation in #1796.