Change package/slice separator in various places to colon from dot.

Allow dots in package names.

Change the internal representation of "package specs" inside Slice to be a
{package,slice} object instead of a "spec" string.

library.getSlices now can take EITHER a "spec" string or a {package,slice}
object.

Linker-created slice files also separate the slice name with a colon (so eg, you
get URLs like "/packages/livedata:tests.js" when running package tests).  (Maybe
this should use a subdirectory instead?)