- split description of package initialization and
  program execution
- better grouping of concerns in section on package
  initialization
- more explicit definition of what constitues a
  dependency
- removed language about constant dependencies -
  they are computed at compile-time and not
  initialized at run-time
- clarified that independent variables are initialized
  in declaration order (rather than reference order)

Note that the last clarification is what distinguishes
gc and gccgo at the moment: gc uses reference order
(i.e., order in which variables are referenced in
initialization expressions), while gccgo uses declaration
order for independent variables.

Not a language change. But adopting this CL will
clarify what constitutes a dependency.

Fixes #6703.

LGTM=adonovan, r, iant, rsc
R=r, rsc, iant, ken, adonovan
CC=golang-codereviews
https://golang.org/cl/99020043