[prebuilt] Revamp prebuilts

scripts/download-prebuilt replaces scripts/download-toolchain and
scripts/download-sysroot, which are both now obsolete stubs that
warn and exec download-prebuilt. The new script uses CIPD when the
tool is available. When not using CIPD, it uses a single file for
all the versions (ZIP file SHA1 checksums) rather than the old
arrangement of many files in many subdirectories.

The new script has --list and --verify modes to describe the state
of the installed prebuilts vs the current versions described in the
file. The makefile check for stale prebuilts is now done via
`scripts/download-prebuilt --verify`.

scripts/update-prebuilt-versions automatically maintains the
prebuilt/zircon.versions file that drives the (non-CIPD) downloads
and the sanity checks. prebuilt/zircon.ensure is maintained by hand
to deploy new versions, and then the script must be run to update
prebuilt/zircon.versions before commit.

ZX-1860 #done
TO-937 #done

Change-Id: Iaf02d2c5f3bdd81008573cd2939b35f1a450398c

[manifest] Move manifest to directory and create a more zircon one

This changes the structure of Jiri manifest to resemble other system
layers. The minimal manifest includes only the repoisoty itself and
hooks while the zircon one also includes other external build
dependencies.

This is a reland of 24a056a4e0d367103644a477cf9dba6523f33a68. The
previous version placed imports in the project section which wasn't
caught by Jiri during validation but failed during build. This change
needs to be coordinated with the change to infra/config and followed
by a manual roll to Garnet to switch to the new manifest location.

Change-Id: Id641afbecc42977de2cc817b368073289aac910e

[manifest] Move manifest to directory and create a more zircon one

This changes the structure of Jiri manifest to resemble other system
layers. The minimal manifest includes only the repoisoty itself and
hooks while the zircon one also includes other external build
dependencies.

Change-Id: I963fce2b4b27ce6ff38a9de5d40cb2b3e2e54c09