date: remove some ambiguity in tzdata references

The use of 'package' in this could be understood to mean a FreeBSD
package provided by pkg, rather than the fact that we use data provided
by IANA. Re-word it to clearly identify tzdata as the IANA Time Zone
Database on first use, then drop subsequent uses of the word 'package'.

Reviewed by: 0mp, pauamma, philip
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D35966