Towards #7401

This can cause unexpected errors in the runtime when done while
scheduler threads are still initializing. Required some restructuring
of the main_args functions in our libraries.

Some minor api and doc adjustments

as per #9112

Closes #9112

Treating a package as the thing that can have other packages depend on it,
and depends on other packages, was wrong if a package has more than one
crate. Now, rustpkg knows about dependencies between crates in the same
package. This solves the problem reported in #7879 where rustpkg wrongly
discovered a circular dependency between thhe package and itself, and
recursed infinitely.

Closes #7879

This will initialize a new workspace.

Install to the first directory in the RUST_PATH if the user set a
RUST_PATH. In the case where RUST_PATH isn't set, the behavior
remains unchanged.

Closes #7402

Closes #8524

rustpkg now accepts most of rustc's command-line arguments and passes
them along to rustc when building or installing.

A few rarely-used arguments aren't implemented yet.

rustpkg doesn't support flags that don't make sense with rustpkg
(for example, --bin and --lib, which get inferred from crate file names).

Closes #8522

rustpkg now uses the workcache library to avoid recompilation.
Hooray!

This commit adds a rustpkg flag, --rust-path-hack, that allows
rustpkg to *search* inside package directories if they appear in
the RUST_PATH, while *building* libraries and executables into a
different target directory.

This behavior is hidden behind a flag because I believe we only
want to support it temporarily, to make it easier to port servo to
rustpkg.

This commit also includes a fix for how rustpkg fetches sources
from git repositories -- it uses a temporary directory as the target
when invoking `git clone`, then moves that directory into the workspace
if the clone was successful. (The old behavior was that when the
`git clone` failed, the empty target directory would be left lying
around anyway.)

`rustpkg build` et al were only checking one directory up to see if it
was in a dir named "src". Ditch that entirely and instead check if the
cwd is descended from any of the workspace paths. Besides being more
intelligent about whether or not something is a workspace, this also
allows for package ids composed of multiple path components.

Now, rustpkg --version does something useful!

The test checks that rustpkg uses the first one, rather than complaining
about multiple matches.

Closes #7241

removed pub on real_args, changed test to use args

This necessitated some cleanup to how we parse library filenames
when searching for libraries, since rustpkg may now create filenames
that contain '-' characters. Also cleaned up how rustpkg passes the
sysroot to a custom build script.

cc #5898

Get rid of special cases for names beginning with "rust-" or
containing hyphens, and just store a Path in a package ID. The Rust-identifier
for the crate is none of rustpkg's business.

This commit allows you to write:

 extern mod x = "a/b/c";

which means rustc will search in the RUST_PATH for a package with
ID a/b/c, and bind it to the name `x` if it's found.

Incidentally, move get_relative_to from back::rpath into std::path

- Made naming schemes consistent between Option, Result and Either
- Changed Options Add implementation to work like the maybe monad (return None if any of the inputs is None)
- Removed duplicate Option::get and renamed all related functions to use the term `unwrap` instead

this has been replaced by `for`

`crate => Crate`
`local => Local`
`blk => Block`
`crate_num => CrateNum`
`crate_cfg => CrateConfig`

Also, Crate and Local are not wrapped in spanned<T> anymore.

`rustpkg build`, if executed in a package source directory inside
a workspace, will now build that package. By "inside a workspace"
I mean that the parent directory has to be called `src`, and rustpkg
will create a `build` directory in .. if there isn't already one.

Same goes for `rustpkg install` and `rustpkg clean`.

For the time being, `rustpkg build` (etc.) will still error out if
you run it inside a directory whose parent isn't called `src`.
I'm not sure whether or not it's desirable to have it do something
in a non-workspace directory.

rustpkg can now build code from a local git repository. In the
case where the local repo is in a directory not in the RUST_PATH,
it checks out the repository into a directory in the first workspace
in the RUST_PATH.

The tests no longer try to connect to github.com, which should
solve some of the sporadic failures we've been seeing.

This should allow fewer symbols to be exported.