Workspace `exclude` doesn't work if nested under a `member` path

Problem

It doesn't appear as though the workspace.exclude key works for paths that are nested under a path included in workspace.members

Steps

1. Create a nested package structure:

/tmp % cargo new outer
     Created binary (application) `outer` package
/tmp % cd outer
/tmp/outer % cargo new middle      
     Created binary (application) `middle` package
/tmp/outer % cd middle
/tmp/outer/middle % cargo new inner
     Created binary (application) `inner` package

2. Add a workspace.members key to the outermost level, and observe that the workspace build succeeds:

/tmp/outer/middle % cd /tmp/outer
/tmp/outer % echo "[workspace]\n  members = [\".\", \"middle\"]" >> Cargo.toml
/tmp/outer % cargo build --all
   Compiling middle v0.1.0 (/tmp/outer/middle)                                                                                                                                                                                                                                            
   Compiling outer v0.1.0 (/tmp/outer)                                                                                                                                                                                                                                                    
    Finished dev [unoptimized + debuginfo] target(s) in 0.34s

3. Now go to the inner package and observe that the build fails:

/tmp/outer % cd /tmp/outer/middle/inner 
/tmp/outer/middle/inner % cargo build
error: current package believes it's in a workspace when it's not:
current:   /tmp/outer/middle/inner/Cargo.toml
workspace: /tmp/outer/Cargo.toml

this may be fixable by adding `middle/inner` to the `workspace.members` array of the manifest located at: /tmp/outer/Cargo.toml

4. Fair enough; let's explicitly exclude this package instead, and try again:

/tmp/outer/middle/inner % cd /tmp/outer
/tmp/outer % echo "exclude = [\"middle/inner\"]" >> Cargo.toml
/tmp/outer % cd /tmp/outer/middle/inner
/tmp/outer/middle/inner % cargo build
error: current package believes it's in a workspace when it's not:
current:   /tmp/outer/middle/inner/Cargo.toml
workspace: /tmp/outer/Cargo.toml

this may be fixable by adding `middle/inner` to the `workspace.members` array of the manifest located at: /tmp/outer/Cargo.toml

It still wants to be in the workspace! We're stuck.

Possible Solution(s)

If there's overlap between paths in members and exclude, I would expect the more-specific path to win. In this case, the excluded path "middle/inner" is more specific than the member paths "." or "middle", so the exclusion should win. This is probably not so simple if the paths contain globs, though.

Notes

Output of cargo version:

% cargo version
cargo 1.31.0 (339d9f9c8 2018-11-16)

Thanks for your looking into this 🦀

Reproduce the nested packages using the outer, middle, and inner Cargo.toml files, then run cargo build from the inner package with workspace.members and workspace.exclude configured as shown. Trace Cargo's workspace member and exclusion resolution; done means the excluded middle/inner package builds independently without the workspace error.