Cargo.lock support, option changes & path prefix fix

[SoulSharer]

As an alternative and compliment to vendoring I've implented support for Cargo.lock crate downloading. https://github.com/panamax-rs/panamax/pull/71

This commit includes several changes:

• Cargo.lock crate download support with --cargo-lock
• Fix for crates path prefixes due to crates starting with uppercase (i.e. "Inflector" crate caused index directories to be created with upper case as in In/fl/..., while cargo tried to download from in/fl/... resulting in 404. (reverted for now)
• Option to skip rustup update with --skip-rustup
• Vendor path is now a long option instead with --vendor_path

It is also no longer needed to clear cargo index for vendor and Cargo.lock crate downloads. They are forced to always download for now.

Example usage

# To skip rustup step which might take a long time
cargo run sync --skip-rustup ./mirror

# Download crates listed in Cargo.lock
cargo run sync --cargo-lock ./Cargo.lock ./mirror

# Sync with vendor
cargo run  sync --vendor_path vendor

[k3d3]

LGTM - Clippy's complaining a bit but I can handle that. Thanks!