cargo_common.bbclass: improve operation with devtool
cargo_common.bbclass: improve operation with devtool

Because devtool inhibits all the normal bitbake fetchers, we can't mask crates.io.
Use EXTERNALSRC to detect if we're being built with devtool, and only mask
crates.io if we're not.

Fixes #195

meta-rust/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust