Converted pkgdetails to mawk; speeds up install by at least 5 minutes.

Also fixes a bug in detecting dependencies; output now matches pkgdetails.c.
This fixes Debian installs (at least the bootstrap phase).

Timed the first part of the install via:
sudo true; time sudo sh -e crouton -d -f/dev/null 2>&1 | grep -q adduser

Results:
Pixel: 5m11s -> 16s
ARM: 7m35s -> 6.5s
Cr48: 9m58s -> 7s

Basically, debootstrap is back to being network-limited as it should be.

Interestingly, the mawk version is generally faster than the c version (up to
2x) due to mawk compiling regexes.