spec: specify order of init() calls

spec: specify order of init() calls

The spec did not specify the order in which
init() functions are called. Specify that
they are called in source order since we have
now also specified the initialization order
of independent variables.

While technically a language change, no
existing code could have relied on this,
so this should not break anything.

Per suggestion from rsc.

LGTM=r, iant
R=rsc, iant, r, ken
CC=golang-codereviews
https://golang.org/cl/98420046