Remove extern crate usage in custom build

[dwijnand]

Is this needed? Let's find out together.

[rust-highfive]

r? @nrc

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[nrc]

Travis seems to be looping in workspaces::you_cannot_generate_lockfile_for_empty_workspaces

[ehuss]

It's actually metabuild_json_artifact which is hanging.

Metabuild was intentionally written at the time to use 2015. If you want to switch it, Target::metabuild_target would need to be changed, though I don't see a reason to.

The hang reveals several bugs, though:

• If metabuild ever fails to build, it generates a message that causes a panic. This should be fixed. I'll try to look at that.
• In some cases, when Cargo panics while processing a job, it hangs forever. I've noticed this in the past, I'll file an issue and look at what it takes to fix.

[dwijnand]

Interesting. Thank you both.