Remove Location, use Url everywhere

Now that rust-url supports windows paths in URLs, this commit jettisons the
janky Location enum in favor of just using Url everywhere.
Remove the update_remotes flag entirely

This will inadvertently fix #337 as no lockfile will imply that all dependencies
are not precise, and will hence be fetched.
Unconditionally write Cargo.lock

Otherwise `cargo-update` ends up not doing much because all sources are
considered equal regardless of their precision.
Update rust-url with a contained bugfix
auto merge of #336 : alexcrichton/cargo/no-more-location, r=wycats

Now that rust-url supports windows paths in URLs, this commit jettisons the
janky Location enum in favor of just using Url everywhere.