Exclude some subdirectories from the package crate

[mulkieran]

Tested by:

1. Running cargo package --all-features.
2. Verifying directories excluded not in target/package/<packagename>
3. cd ../target/package/<packagename>.
4. Run cargo build --features=vendored.
5. echo $? yields 0.

Also run cargo test with excellent results.

[mulkieran]

@diwic I think this ought to be an extremely safe change. It excludes only files that are part of the dbus source tree pulled in by the git submodule. And exclude won't fail if directories that are excluded have stopped existing so it should work pretty nicely even if the dbus source tree changes.

[diwic]

Thanks!