SI-4742 Make -Xcheckinit aware of constants.

Members defined as `final val x = <literal>` are given
a ConstantType. The constant is folded into the accessor
method `x`, and the field itself is never initialized.
(Related discussion: SI-4605)

As such, -Xcheckinit spuriously warns when calling that
accessor.

This commit disables the checks for constants.

This will also fix the checkinit build (failure tracked as SI-7839),
which is the victim of this a spurious scolding.
Merge pull request #2982 from retronym/topic/checkinit

SI-4742 Make -Xcheckinit aware of constants.
fix minor regression in quasiquote reification

emptyValDef should always be reified as emptyValDef. After the the
introduction of SyntacticValDef this ceased to be true.
harden test case to avoid parsing failure when name is keyword
refactor out common logic between various statSeq-s in parser

Partial functions are left outside of the call to statSeq to allow
re-use and overriding capabilities for quasiquotes.
refactor out range position utility constructors

This will help to re-use same logic for creating range positions even
if atPos isn't used to assign position to the tree.
SI-6841 SI-6657 add support for packages into quasiquotes and toolbox

In order to implement this a new parser entry point
`parseStatsOrPackages` that augments current parseStats with ability
to parse "package name { ... }" syntax.
minor changes due to typos
thickets wip