Dump dependency information during the build

[neonichu]

• Extend BuildDependencyInfo with module/header dependencies
• Extend ValidateDependenciesTaskAction to dump per target dependency information in the new format
• The new behavior is opt-in behind a DUMP_DEPENDENCIES build setting

[neonichu]

@swift-ci please test

[bob-wilson]

It's hard to compare the dependency info format changes here. When the new setting is not enabled, will this produce the same output format as before?

[neonichu]

When the new setting is not enabled, will this produce the same output format as before?

This adds a new key dependencies as a peer to the existing inputs one, but doesn't change the format of any existing keys otherwise. Versioning is probably a good idea either way, but my guess is that extra keys are typically ignored by parsers, so this change should be forward compatible.

I don't have a clear idea how the format should look like yet, so I was thinking to make additive changes like this at first and once we have a clearer picture, we could try to consolidate the existing structure if needed.

[neonichu]

@swift-ci please test

[neonichu]

@swift-ci please test

[neonichu]

Versioning is probably a good idea either way

Actually, we won't need a version here, yet, since we don't serialize any dependencies as part of the existing command. The new format is only utilized for the output generated during the build.