This changeset makes it possible to build the Second Life viewer using
Python 3. It is designed to be used with an equivalent Autobuild branch
so that a developer can compile without needing Python 2 on their
machine.

Breaking change: Python 2 support ending

Rather than supporting two versions of Python, including one that was
discontinued at the beginning of the year, this branch focuses on
pouring future effort into Python 3 only. As a result, scripts do not
need to be backwards compatible. This means that build environments,
be they on personal computers and on build agents, need to have a
compatible interpreter.

Notes

- SLVersionChecker will still use Python 2 on macOS
- Fixed the message template url used by template_verifier.py

The nghttp2 autobuild package has copyright information that embeds a newline.
autobuild install --copyrights correctly produces that information onto two
lines. But that means packages-formatter.py must process any lines that do not
match its expected 'packagename: copyright' pattern as the continuation of the
preceding package's copyright information.

Since the processing for autobuild install --versions is so very similar, fold
both into the same outer loop.

Also report all duplicates for any package, instead of stopping at the first.

Ensure that AUTOBUILD is always in native path syntax on every platform.
Remove local build.sh logic to infer AUTOBUILD: either the generic build.sh
has set it properly, or scream for help.
Since cygwin bash scripts need a cygwin-syntax path to autobuild, convert to a
local 'autobuild' variable using shell_path.
Remove kludges in other languages involving cygdrive twiddling.

Hopefully this will make certain errors clearer, notably failure to launch
autobuild due to "cannot find the file specified".