Add `--extern-loc` to augment unused crate dependency diagnostics

This allows a build system to indicate a location in its own dependency
specification files (eg Cargo's `Cargo.toml`) which can be reported
along side any unused crate dependency.

This supports several types of location:
 - 'json' - provide some json-structured data, which is included in the json diagnostics
     in a `tool_metadata` field
 - 'raw' - emit the provided string into the output. This also appears as a json string in
     `tool_metadata`.

If no `--extern-location` is explicitly provided then a default json entry of the form
`"tool_metadata":{"name":<cratename>,"path":<cratepath>}` is emitted.

0 files changed, 0 insertions, 0 deletions