build: Switch to argparse for argument parsing

Signed-off-by: Euan Harris <euan.harris@citrix.com>
build: Add --external-dependencies flag

Signed-off-by: Euan Harris <euan.harris@citrix.com>
XenBuilder integration: Include external dependencies in cache hashes

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #73 from euanh/external-dependencies

Include external dependencies when calculating package hashes for caching