Add a way to tell rustpkg not to install all the crate files in a package

[catamorphism]

@metajack pointed out that a library might have a main crate, and then a src/examples directory containing lots of subdirectories with main.rs files. If you rustpkg install, you probably don't want to install the example programs. Maybe we could add another "magic" filename for files we want to build but not install. Or maybe there's another solution.

[steveklabnik]

I also picked up on this pattern myself, so examples could be a good name.

[catamorphism]

I think this will fall out of my work on #7879 but I'm not totally sure. In particular: there's a question of, does building a package mean building every crate file in any subdirectory of the package directory? Or, does it mean building the crates in the top-level directory, plus all their dependencies (searching subdirectories of the package dir as well as the RUST_PATH)? If the latter, then this bug will become a non-issue, but if the former, it's still an issue.

[catamorphism]

Closely related to #7240

[jhasse]

What do you think about having the main crate files simply be put in the src directory? For example for glfw:

glfw-rs/src/lib.rs // #[crate_id = "github.com/bjz/glfw-rs#glfw:0.1"];
glfw-rs/src/examples/window/main.rs // extern mod glfw;

Executing rustpkg build or rustpkg install would then only install this "main" crate (rustpkg build glfw would also still work).

Also this removes the repeated name. Let's say I have a project foobar and am using rustpkg for building it. /home/jhasse/projects/foobar/src/foobar/main.rs repeats the name. This was pointed out by someone on IRC a couple of weeks ago as being a "bug" of rustpkg.

The package id would then also be an optional argument for install and build. This way editors can execute rustpkg in any project without extracting the crate name.

[emberian]

rustpkg is gone.