Proposal: Remove source tracking from setup

[Robert Collins]

I'd like to remove all source package tracking from setup.exe.

By this I mean:
* Do not log downloaded sources to installed.db
* skip parsing any existing source records from installed.db.
* Continue to allow sources downloading.

At the same time I'd like to add a header record to installed.db -
something like
INSTALLED.DB VERSION 2

to allow versioned upgrades in the future.

This will silently break any current scripts or tools like cygcheck that
parse installed.db - which is why I'm raising it here. OTOH it will
allow detection of further breakage.

The goal of this change is to
1) make setup.exe a little simpler.
2) take a step towards allowing sources to be downloaded to arbitrary
directories.

Rob