With the new output mode --output=files, cquery lists all files advertised by the matched targets in the currently requested output groups.

This new mode has the following advantages over --output=starlark combined with an appropriate handcrafted --starlark:expr:

• provides a canonical answer to the very common "Where are my build outputs?" question
• is more friendly to new users as it doesn't require knowing about providers and non-BUILD dialect Starlark
• takes the value of --output_groups into account
• stays as close to the logic for build summaries printed by bazel build as possible

Fixes #8739

RELNOTES: cquery's new output mode --output=files lists the output files of the targets matching the query. It takes the current value of --output_groups into account.