update sysutils/packer to 1.0.2

o update Packer to 1.0.2
o fetch from Hashicorp
o switch to USES=go
o inline single-entry plist
o add LICENSE_FILE
o take maintainership

Reviewed by: swills
Approved by: swills (maintainer)
Differential Revision: https://reviews.freebsd.org/D11408