Description

This is a follow-up to #2498 (comment).

Rebar3 currently attempts to fetch and compile optional transitive dependencies, even when:

• they are only declared as optional in mix.exs,
• they are not listed in rebar.config,
• and they are Mix-only packages.

This causes build failures because Rebar3 does not understand Mix deps out of the box and should be ignoring them entirely.

Example

Repo: https://github.com/erlef/purl

• rebar.config: no deps
• mix.exs: stream_data listed as optional
• Expected: Rebar3 ignores it
• Actual: Rebar3 tries to compile stream_data and fails

Expected behavior

Optional dependencies not present in rebar.config should be ignored by Rebar3, even if they appear in mix.exs. This is needed for cross-tool compatibility when a library supports both build tools.