pkg2: make "REQUIRE" the default argument to parse_requires.
pkg2: allow fixed version discovery handle unknown SHA1s smoothly.
pkg2: add .julia directory finding and creation.

I'm not sure that we still want .julia to always be a git repo now,
but for the time being, it's easiest to just steal code from Pkg 1.
In any case, the business of prompting users for their GitHub info
is a complete annoyance and has to go. We should probably get rid
or that in the current Pkg, but I'm loathe to change anything there.
Merge branch 'master' of github.com:JuliaLang/julia