package: Switch debug source handling to use prefix map

package: Switch debug source handling to use prefix map

Reproducible builds are no longer a configuration option but are required.
We also rely on the prefix mapping capability of the compilers now.

As such, rewrite the source locating code to use the prefix maps instead
of taking a guess about WORKDIR which isn't correct for kernels, gcc,
externalsrc and probably more.

Instead, iterate the maps to locate any matching source code, keeping
in mind that multiple maps may map to one target location.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>