depends: Consistent use of package variable

Summary:

All other mk files use the package variable consistently except for the
two instances here, which have always been here, since depends was
introduced in 0.10.

Backport of core PR17928.

Test Plan: Run the Gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5668