package: Create global package file list and use throughout PACKAGEFUNCS

package: Create global package file list and use throughout PACKAGEFUNCS

Currently we do a signficant amount of tree traversal in many different places
which in inefficient. We can assume that the files don't change and cache the
file list which gives an efficiency improvement which this patch does using
a global variable.

(From OE-Core rev: 2d7608842d2dab07065e60aab729a5c8fd6b7907)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>