build-system/cargo: refactor phases to successfully build

* guix/build-system/cargo.scm (%cargo-build-system-modules):
Add (json parser).
(cargo-build):
[vendor-dir]: Define flag and pass it to builder code.
[cargo-test-flags]: Likewise.
[skip-build?]: Likewise.
* guix/build/cargo-build/system.scm (#:use-module): use (json parser).
(package-name->crate-name): Delete it.
(manifest-targets): Add it.
(has-executable-target?): Add it.
(configure): Add #:vendor-dir name and use it.
Don't touch Cargo.toml.
Don't symlink to duplicate inputs.
Remove useless registry line from cargo config.
Define RUSTFLAGS to lift lint restrictions.
(build): Add #:skip-build? flag and use it.
(check): Likewise.
Add #:cargo-test-flags and pass it to cargo.
(install): Factor source logic to install-source.
Define #:skip-build? flag and use it.
Only install if executable targets are present.
(install-source): Copy entire crate directory not just src.
[generate-checksums] pass dummy file for unused second argument.
(%standard-phases): Add install-source phase.

Signed-off-by: Chris Marusich <cmmarusich@gmail.com>