"rustpkg build foo" does not work properly with sub directories

[erickt]

rustpkg build ... with subdirectories overwrites binaries. Here's an example of the problem:

% mkdir -p src/a/b src/a/c
% echo 'fn main() { println!("b"); }' > src/a/b/main.rs
% echo 'fn main() { println!("c"); }' > src/a/c/main.rs
% rustpkg build a
% find build
build/x86_64-apple-darwin
build/x86_64-apple-darwin/a
build/x86_64-apple-darwin/a/a
% ./build/x86_64-apple-darwin/a/a
c

The workaround is to explicitly specify the sub executable:

% rustpkg build a/b
% rustpkg build a/c
% find build
build/x86_64-apple-darwin
build/x86_64-apple-darwin/a
build/x86_64-apple-darwin/a/b
build/x86_64-apple-darwin/a/b/b
build/x86_64-apple-darwin/a/c
build/x86_64-apple-darwin/a/c/c
% ./build/x86_64-apple-darwin/a/b/b
b
% ./build/x86_64-apple-darwin/a/c/c
c

cc @catamorphism

[emberian]

rustpkg is gone