CI/xtask for Validating State of Dependency Tree

[cwfitzgerald]

cargo tree is a very useful tool for evaluating the dependency tree on a given platform, package, and feature set.

We should build a set of queries that tests for/against the presence of dependencies on certain platforms in certain situations. There's no obvious programmatic output, but a regex done in an xtask would be plenty.

Discovered as part of the absolute chaos that is #6980 and to validate #6949. Possible queries:

• cargo tree -p wgpu --target wasm32-unknown-unknown -e normal does not contain wgpu-core
• cargo tree -p wgpu --target wasm32-unknown-unknown --features wgsl,spirv -e normal does not contain wgpu-core
• cargo tree -p wgpu --target x86_64-pc-windows-msvc -e normal does not contain web-sys
  etc.

[kpreid]

Instead of parsing cargo tree, how about using cargo deny? You would need a separate config file for each "query", but it would do the job.

[cwfitzgerald]

It seems a bit heavy handed imo - all I'm thinking of doing for "parsing" here is substring searches or maybe a light regex, plus it's nice it's included in the install of cargo.