Auto merge of #72603 - jsgf:extern-loc, r=nikomatsakis

Implement `--extern-location`

This PR implements `--extern-location` as a followup to #72342 as part of the implementation of #57274. The goal of this PR is to allow rustc, in coordination with the build system, to present a useful diagnostic about how to remove an unnecessary dependency from a dependency specification file (eg Cargo.toml).

EDIT: Updated to current PR state.

The location is specified for each named crate - that is, for a given `--extern foo[=path]` there can also be `--extern-location foo=<location>`. It supports ~~three~~ two styles of location:
~~1. `--extern-location foo=file:<path>:<line>` - a file path and line specification
1. `--extern-location foo=span:<path>:<start>:<end>` - a span specified as a file and start and end byte offsets~~
1. `--extern-location foo=raw:<anything>` - a raw string which is included in the output
1. `--extern-location foo=json:<anything>` - an arbitrary Json structure which is emitted via Json diagnostics in a `tool_metadata` field.

~~1 & 2 are turned into an internal `Span`, so long as the path exists and is readable, and the location is meaningful (within the file, etc). This is used as the `Span` for a fix suggestion which is reported like other fix suggestions.~~

`raw` and `json` are for the case where the location isn't best expressed as a file and location within that file. For example, it could be a rule name and the name of a dependency within that rule. `rustc` makes no attempt to parse the raw string, and simply includes it in the output diagnostic text. `json` is only included in json diagnostics. `raw` is emitted as text and also as a json string in `tool_metadata`.

If no `--extern-location` option is specified then it will emit a default json structure consisting of `{"name": name, "path": path}` corresponding to the name and path in `--extern name=path`.

This is a prototype/RFC to make some of the earlier conversations more concrete. It doesn't stand on its own - it's only useful if implemented by Cargo and other build systems. There's also a ton of implementation details which I'd appreciate a second eye on as well.

~~**NOTE** The first commit in this PR is #72342 and should be ignored for the purposes of review. The first commit is a very simplistic implementation which is basically raw-only, presented as a MVP. The second implements the full thing, and subsequent commits are incremental fixes.~~

cc `@ehuss` `@est31` `@petrochenkov` `@estebank`

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