Update fork

As stated in rust-lang#8486 it would help to the discovery of the
builtin aliases the facto of printing them with the
`cargo --list` command.

- Extracted the builtin aliases currently implemented to a
sepparated `const`.
- Make all of the functions that interact with these aliases
point to that function.
- Refactored the `list_commands` fn in order to include with the
builtin and external commands, the builtin aliases that come with
cargo by defaut.

Added a test that checks that the aliases that currently
are builtin with cargo are indeed being printed with the rest
of the commands when `cargo --list` is called.

Closes rust-lang#8486

As @ehuss correctly suggested, we could just declare
in one `const` structure for every builtin alias the
following: `(alias, aliased_command, description)`.

Therefore, the suggestion has been applied and the
`BUILTIN_ALIASES` const has been refactored.

Also, the `builtin_aliases_execs` now parses the
`BUILTIN_ALIASES` const searching for a
"possible alias command" returning an option with the
previous info structure or `None`.