Fixes rust-lang#5650.  cc rust-lang#5435

As part of my recent work on profiles, I introduced some situations where a
library can be compiled multiple times with different settings.  Doctests were
greedily grabbing all dependencies for a package, regardless of which target
is was for.  This can cause doctests to fail if it links multiple copies of
the same library.

One way to trigger this is `cargo test --release` if you have dependencies, a
build script, and `panic="abort"`.  There are other (more obscure) ways to
trigger it with profile overrides.

In rare cases, the profile_for is set incorrectly for this temporary unit used
to compute dependencies.  However, it's not needed since the RunCustomBuild
Unit's parent is already in the map.

- Consolidate logic for determining doctest dependencies.
- Use a struct for the doctest info in `Compilation`.