Auto merge of #3562 - matklad:member-workspace, r=alexcrichton

Auto merge of #3562 - matklad:member-workspace, r=alexcrichton

Find workspace via `workspace_root` link in containing member

This PR proposes to change the logic for determining workspace members. Here are the algorithms we used previously for this:

# [RFC](https://github.com/rust-lang/rfcs/blob/master/text/1525-cargo-workspace.md)  flavor

If `[members]` key present, it compliantly defines the set of members. Otherwise, members are all (transitive) path dependencies.

The problem with this approach is that it violates convention over configuration principle: almost always you want a path dependency to be a member, even if there are some explicit members. Listing **all** path deps is just to much work.

# Original implementation

So, the actual algorithm **unconditionally** included path dependencies as memebers.

This is also problematic, because certain workspace configurations become impossible. In particular, you can't have a path dependency which is not a member of the workspace. This issue was reported in #3192.

# Current implementation

Current implementation (was merged couple of days ago) includes path dependency into the workspace only if is inside the workspace directory. This solves the problem in #3192 perfectly. However, some configuration are still impossible: you can't have a non-member path dependency inside a workspace directory. But the thinking is that if you don't want this path-dep to be a member, just don't put it inside the workspace directory.

There is another problem with current imlementation. Suppose you have an explicit member which lives alongside the workspace. Suppose this member has a path-dep which lives inside the member's folder. Under current implementation, this path-dep won't be a member of the workspace. It seems logical that it should be though (but we haven't received any actual bug reports yet)!

# Implementation in this PR

So, with this PR, the logic is as follows: members are explicit members + all path dependencies which reside under any of the explicit members.