(maint) Lift root out of inner loop into outer loop

root will never change, so setting it inside the loop to a new Pathname object
each time serves no purpose. This commit lifts it out into the outer loop in
that method.
(maint) Set a Pkg::Config variable for packaging_root

PACKAGING_ROOT is set as a constant when invoked using packaging.rake, but no
Pkg::Config variable previously existed to record this information. This commit
adds a packaging_root variable that mirror project_root in several ways.
Pkg::Config::packaging_root defaults to PACKAGING_ROOT if it set, or if not
set, it will be set to the parent directory of lib, which tends to be
ext/packaging.
(RE-1161) Expand templates before templatizing

Previously, templates could be globs that would expand to multiple files. This
was lost in the librarification of packaging, and this commit returns that
functionality. Templates are first expanded, flattened and filtered before
being evaluated later in tar.rb. This allows globs to be successfully used in
the templates field of the project_data.
Merge pull request #279 from haus/fix_the_template_globbing

(maint) Expand templates before templatizing