package.bbclass: improve -dbg and -src package ordering

package.bbclass: improve -dbg and -src package ordering

nativesdk-gpgme fails package_qa when setting PACKAGE_DEBUG_SPLIT_STYLE
= "debug-with-srcpkg".

ERROR: nativesdk-gpgme-1.10.0-r0 do_package_qa: QA Issue: non debug package contains .debug directory: nativesdk-python3-gpg path /work/x86_64-nativesdk-oesdk-linux/nativesdk-gpgme/1.10.0-r0/packages-split/nativesdk-python3-gpg/usr/local/oecore-x86_64/sysroots/x86_64-oesdk-linux/usr/lib/python3.5/site-packages/gpg/.debug/_gpgme.cpython-35m-x86_64-linux-gnu.so [debug-files]

This turns out to be because the automatic moving of the -dbg package to
the beginning of the package list is disabled in that case, so the
python3-gpg packages that the recipe prepends to PACKAGES ends up before
the -dbg package.

It's not clear why the "and not split_source_package" was added when
debug-with-srcpkg was introduced. Presumably the intention was to
prevent the -dbg package to end up before the -src package, which we of
course need to. But at the same time, we still need -dbg packages to end
up before all other packages.

Using list.insert(0, ...) also means that if there happens to more than
one -dbg package, their relative ordering gets inverted in the new list.

This tries to fix these issues by sorting the packages by (priority,
original position), where priority is 10 for -src, 30 for -dbg and 50
for everything else. That guarantees that packages of the same "type"
preserve their relative ordering, while also ensuring that -dbg always
preceed other packages. This scheme is also quite extensible, and,
should the need arise, one could even expose the priorities as a knob
the recipe author could use to ensure specific orderings of packages
instead of the somewhat fragile and coarse-grained method of "prepend or
append, and ensure you do that in a proper order".

Probably the autodebug condition needs to stay, but I think the
split_source_package condition in the preceding elif should be removed,
so that that logic applies to all packages called -src, not just the one
we might have created a few lines above.

(From OE-Core rev: 805edbc7dc9ceae00d991f9b4e185bbbe1d3ba45)

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>