cleanup toward eliminating package global name space

  * switch to real dot (.) instead of center dot (·) everywhere in object files.
    before it was half and half depending on where in the name it appeared.
  * in 6c/6a/etc identifiers, · can still be used but turns into . immediately.
  * in export metadata, replace package identifiers with quoted strings
    (still package names, not paths).

R=ken2, r
CC=golang-dev
https://golang.org/cl/190076