Implement building packages from bzr/git/hg/svn/etc

I added code to fetch sources in ${XBPS_BUILDDIR} with your custom command. This is how it works:

In a build template "templates/foo/template" you remove $distfiles and $checksum.
The "nofetch" and "noextract" variables must be set and a function must be used to fetch
the sources, like:

do_fetch()
{
  echo "Fetching sources via git repository..."
  git clone git://foo.or.cz/blah.git"
}

For example to make a build template for XBPS that fetches the sources from bzr latest
code is the following:

---- templates/xbps-snapshot-bzr ---

pkgname=xbps-snapshot-bzr
build_style=gnu_makefile
version="$(date +%F)"
make_install_target="..."
short_desc="..."
long_desc="..."

nofetch=yes
noextract=yes
conf_files="..."

Add_dependency ...

do_fetch()
{
    msg_normal "Fetching HEAD bzr branch from launchpad..."
    bzr branch lp:xbps ${pkgname}-${ersion}
}

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Also don't forget that you can do everything you need if you use "build_style=custom-install".
fetch, configure, build and install is all handled in the "do_install()" function.