See if we can exclude some extraneous files from upstream crates

[mulkieran]

In particular:

• backtrace-rs: ci dockerfiles
• libm: ci dockerfiles
• libdbus-sys: dockerfiles in vendor subdir

[mulkieran]

• Exclude ci directory from packaged crate rust-lang/backtrace-rs#555
• Exclude ci directory from packaged crate rust-lang/libm#282
• Would you consider a new informational advisory class, "distributes-executable"? rustsec/advisory-db#1737
• Add advisory for executable in serde_derive crate rustsec/advisory-db#1738
• Exclude some subdirectories from the package crate diwic/dbus-rs#446

[mulkieran]

We do not currently depend on backtrace, but will depend on it if we run a cargo-update and update tokio. But, currently, backtrace will be replaced by a stub by vendor-filterer anyway.

[mulkieran]

I think we can close this. All PRs are either merged or rejected, and the one issue is under heavy discussion and not really an action item.