build: handle --root feeds script feature

build: handle --root feeds script feature

Rework the package SOURCE entry handling to account for the --root feeds
script feature.

Move the SOURCE entry string manipulation logic outside package-defaults.mk
in package.mk and limit only to non DUMP scenario to not pollute the .mk
too much.

Restructure the previous logic and add a new additional condition.
If we detect the package comes from a feed, replace any feed path that
have the _root prefix to the feed name with the non-root variant (the
feeds script create a symbolic link to it) and point the package SOURCE
entry to what the symbolic link points to.

Example:
Feed link: feeds/base_root/package -> feeds/base
Package: feeds/base_root/package/system/uci -> feeds/base/system/uci

Link: https://github.com/openwrt/openwrt/pull/20459
Signed-off-by: Christian Marangi <[email protected]>