git: Add support for project cache directory

This update introduces support for setting up a git clone alternate
reference location. The SHRINKWRAP_PROJECT_CACHE environment variable
specifies the mirror directory location.

Both a bare clone and a full clone can be used as a reference
repository. Additionally, a new component attribute, "project", allows
naming the project cache directory. For example:

kvmtool:
   project: kvmtool
   remote: https://git.kernel.org/pub/scm/linux/kernel/git/will/kvmtool.git


   revision: master

Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>