Limit the listed dependencies to those that are actually reachable

[bruceg]

The data source underlying rust-license-tool lists all entries in the dependency tree, even if they can't be reached under any actual combination of top-level feature flags. While we can't reasonably expand every possible combination of the top-level feature flags (Vector has 228 which leads to about 10^440 unique combinations), we should be able to reduce that dependency tree by producing sets of possible feature flags for each crate and thereby eliminate sub-dependencies that can't be turned on. This would reduce the size of the generated licenses file, producing a more accurate reflection of what components are actually used.

[jszwedko]

Is rust-lang/cargo#10801 related to this?

[bruceg]

Yes, that's the one. I was looking for that link when I wrote the above but couldn't find it. There may be opportunities for us to solve this without support from that issue, however.