This patch refactors the cargo-build-system builder code to correctly build
imported Rust crates. Specifically the changes:

* Do not rely on a Cargo.lock presence to determine installation. Cargo will
automatically create the file at its first invocation, so instead we check
the manifest for any executable targets before attempting an installation
* Do not attempt to modify the Cargo.toml file. There are many system specific
package in crates.io (e.g. for Windows, Redox, Fuschia, WASM, etc.) and
attempting to keep up with what crates must be patched out is futile.
* The build phases will honor a skip-build? flag which allows for
short-circuiting for optional packages which require nightly features or cannot
be built for the current platform.

Changes which still need to be done:
* Update the host-side code to expand transitive inputs: cargo requires that
all transitive crate dependencies are present in its (vendored) index, but
doing so by hand in the package definitions will become unwieldy.
* Update the host-side code to detect any "circular" dependencies which can
result from a naive import

Unfortunately there isn't a good way to test this patch at the moment.
Importing a non-trivial crate requires a lot of manual resolution, especially
with the points above remaining unimplemented.

If someone would really like to see the input package definitions I was using
to test, I'd be happy to share, though I'd advise that it's pretty hacked up
for my own convoluted testing at the moment.

—Ivan