rustc: fix use of shared source for new poky/oe-core which wipes ${S}

The oe-core/poky change 'base.bbclass wipe ${S} before unpacking
source' (poky: a56fb90dc3805494eeaf04c60538425e8d52efc5, oe-core:
eccae514b71394ffaed8fc45dea7942152a334a1) wipes ${S} in do_unpack prior
to unpacking. This breaks our shared source as we set ${S} to the shared
location, and don't actually unpack anything (the result is we try to
build rustc without any source code, which fails predictably)

Avoid this by clearing do_unpack[cleandirs].

At the same time take pieces of gcc-shared-source.inc and note how & why
we differ from how gcc operates.

This is a bit of a hack, and only happens to work because we know the
exact method that do_unpack uses to clear ${S}, and using python()
happens to get called at the "right time".

classes/shared-source-use.bbclass

@@ -1,3 +1,23 @@
+# gcc's shared source code disables fetch (via the commented code below). We
+# can't do that because rust.bb currently fetches a rustc-snapshot archive itself.
+#do_fetch() {
+#	:
+#}
+#do_fetch[noexec] = "1"
+
+# gcc does `deltask` do_unpack. We avoid this so that the depends work sanely
+# (things that need source code can still be ordered after do_unpack).
+# As a side effect, we can also unpack things that aren't shared.
+# Note: just setting this normally doesn't work. Use of python() is required.
+python () {
+	d.setVarFlag('do_unpack', 'cleandirs', '')
+}
+# Avoid disabling do_patch for the same reason.
+#deltask do_patch
+
+SRC_URI = ""
+
 S = "${TMPDIR}/work-shared/${SOURCE_NAME}-${PV}-${PR}"
 
 do_unpack[depends] += "${SOURCE_NAME}-source-${PV}:do_patch"
+do_populate_lic[depends] += "${SOURCE_NAME}-source-${PV}:do_unpack"