support PRODUCT_SOURCE_ROOT_DIRS product variable

Soong analyzes the entire source tree even though not every lunch target
needs to know about every module. For example, OEM sources can be
ignored for cuttlefish products. This functionality allows blueprint to
ignore a list of undesired directories.

Bug: 269457150
Change-Id: I1eec5d7b6a268cae4c633d8d89ed485598ebca45