Add --default-target flag to cargo fetch

[zopsicle]

Problem

The documentation for cargo fetch reads:

If --target is not specified, then all target dependencies are fetched.

I only need dependencies for the default target. With “default target” I mean the target that a plain cargo build would assume.

Proposed Solution

Add a --default-target option to cargo fetch that would be equivalent to --target <default target>, e.g. --target x86_64-unknown-linux-gnu if cargo build would assume x86_64-unknown-linux-gnu.

Notes

#9423 seems similar but for cargo metadata.