Transitive minor updates of deps shouldn't fail `cargo update`

[alexcrichton]

Let's say you've got the following project structure:

• Top level project, A
• A depends on log = "0.3.6"
• A depends on B, a sub-project in this same directory
• B depends on log = "0.3.6"

When you generate a lock file for this project let's say you get log = "0.3.6". If you then later edit B's Cargo.toml to instead say log = "0.3.8" then if you run cargo update -p a Cargo will inform you:

error: failed to select a version for `log` (required by `b`):
all possible versions conflict with previously selected versions of `log`
  version 0.3.6 in use by log v0.3.6
  possible versions to select: 0.3.8

Cargo should gracefully handle this and not generate an error!

I'm not sure whether this is 100% related to path dependencies. Some more info can also be found here.

[upsuper]

FWIW this happened again for https://hg.mozilla.org/integration/autoland/rev/6b5e1e49a4c2fdaa164072ceef196c66197cf1d5

error: failed to select a version for `app_units` (required by `style`):
all possible versions conflict with previously selected versions of `app_units`
  version 0.5.1 in use by app_units v0.5.1
  possible versions to select: 0.5.2