Adds another argument to readTree itself which can be passed when
importing readTree (e.g. in our default.nix) to filter the arguments
passed to a target based on that target's location in the tree.
This is intentionally not yet mentioned in the docs, and also
intentionally implemented in such a way that the API surface of
readTree doesn't change. The reason for this is that I want to figure
out whether these filter functions are actually useful, e.g. within
depot by filtering user-folder passing, and then refactor the readTree
API to find a public way of exposing this as part of the readTree
function itself (and not its import).
Relates to b/143.
Change-Id: I2cdf09f67916527d2337f4bfb578749aeac51a6a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3433
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>