Support for PkgCreator to not rebuild a package on every run

- Contribution by Nick McSpadden and some additional changes
- Squashed commits below:

PkgCreator: Clarify output when finding a matching package. (+11 squashed commits)
Squashed commits:
[1a93e4c] PkgCreator: pkg_path is based on request's pkgdir, not always the cache dir
[d2133c7] PkgCreator: Define new_package_request in output vars
[d86370c] PkgCreator: Support for 'force_pkg_build' input variable
[d966809] PkgCreator: set 'new_package_request' that autopkg tool can check for reporting
[4adf77f] PkgCreator: Handle case of a missing PackageInfo file
[fe4437a] PkgCreator: minor cleanup
[96426e5] Remove old reference in xarExpand
[e1e3fdc] PkgCreator: do the rest of the existing pkg check only if we found a pkg
[5b4aa30] Fixed formatting issues that made diffs useless
[05cd339] Fixed spaces-tabs
[1389c81] Added idempotency check

Checks name of package, then bundle id + version to see if there's a
match.
Update CHANGELOG.md
Added the script that generates processor documentation
Merge branch 'master' of https://github.com/autopkg/autopkg