cache: Add caching wrapper around mock

Designed as a drop-in replacement for mock in a makefile.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
cache: Add PLANEX_CACHEDIR environment variable

Signed-off-by: Euan Harris <euan.harris@citrix.com>
cache: Include the mock configuration file in the package hash

Signed-off-by: Euan Harris <euan.harris@citrix.com>
cache: Move build outputs into a subdirectory to make way for cache metadata

Signed-off-by: Euan Harris <euan.harris@citrix.com>
cache: Make it easier to identify our temporary directories

Signed-off-by: Euan Harris <euan.harris@citrix.com>
cache: Add defaults for intercepted command line arguments

Signed-off-by: Euan Harris <euan.harris@citrix.com>
cache: Add --debug option

Signed-off-by: Euan Harris <euan.harris@citrix.com>