devel/py-amalgamate: Fix data_files install location

• Install data_files into ${DOCSDIR} instead of ${PREFIX}
• Allow concurrent installation for bin/amalgamate.py and ${DOCSDIR}

PR: 261159
Approved by: lwhsu (python)
MFH: 2022Q1 (ports compliance)

(cherry picked from commit 33e2061728d30f5e43912b604da958ede94ba59f)