fix({build,package}.sh): log previously installed pacdeps as such

[oklopfer]

Purpose

if I install a pacdep manually, it loses connection to its parent, and is not properly logged as a pacstall depends in metadata.

Approach

in build.sh, check if existing metadata lists this package as a pacdep; if so, log it as one.
in package.sh, if the package is already installed, check if it has the declaration in its metadata; if not, add it

Progress

• do it
• test it

Addendum

part of pacstall/pacstall-programs#5986

Checklist

• I confirm that I have read the contributing guidelines, and this pull request is abiding by all the clauses stated in the guideline.