(RE-2467) Move shared code into Pkg::Util::File

Both the tar and gem workflows copy files from the project into the
workdir using very similar means. This commit abstracts that logic into
a utility method called install_files_into_dir. The utility copies all
files in the project root that match a given pattern into the workdir.
(RE-2467) Add tests for the install_files_into_dir method

This adds a simple and convoluted test for the install_files_into_dir
method that ensures that the correct behavior is seen for files,
symlinks, directories, and empty directories.
Merge pull request #310 from haus/ticket/master/re-2467-move-common-behavior-into-file-utility

(RE-2467) Move shared code into Pkg::Util::File