.pytool/Plugin/EccCheck: Add PACKAGES_PATH support

.pytool/Plugin/EccCheck: Add PACKAGES_PATH support

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4326

EccBuild currently assumes all packages reside in WORKSPACE. However,
this is obviously not the case for many setups. Most notably, Ext4Pkg
is located in edk2-platforms/Features and thus cannot be in WORKSPACE
in any reasonable setup.

Use Edk2Path to locate the package in WORKSPACE and PACKAGES_PATH.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Signed-off-by: Marvin H?user <mhaeuser@posteo.de>
Acked-by: Pedro Falcato <pedro.falcato@gmail.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>