Auto merge of #6363 - dwijnand:add-executable-in-json-output, r=matklad

Include executable in JSON output.

Fixes #5426
Rebase of @patriksvensson's #5517
CC @matklad

I didn't really get into the issue or the code, I just interatively rebased Patrik's branch and then massaged and cleaned up the code until the tests passed.  So please double check it for code correctness, test case correctness and test case coverage.

Particularly the branch changed an if condition according to [this suggestion](https://github.com/rust-lang/cargo/pull/5517#issuecomment-388557248) by Aleksey.  I rolled that back because at one point it helped fix a series of tests.  But let me know if that should be included here.