Add initial support for layer metadata

for now, we add just collection of recipe's layer full path on disk.

The basename of that path could be used as layer's name, and the full path can be used to check which source files belong to recipe's layer and which ones are overrides from other layers (by checking if rootpath starts with the full path of the layer). But we leave that to a4f harvest, for now.

For each recipe, we collect layer's local path, as well as remote git repo url and revision (if available).

Moreover, for each local SRC_URI, we check if it comes from the same layer as its recipe (in such case we don't record any additional metadata), or if it's an override from another layer (in such other case we record the overriding layer).