Use apt-get indextargets if available.
Use apt-get indextargets if available.

This improves the handling of many edge-cases with regards to locating
the correct Sources files on the local system.

See https://bugs.debian.org/801594 for context.

Tested by verifying that on my machine, both code paths result in the
same |sourcesPaths| value:

$ LD_LIBRARY_PATH=/tmp/usr/lib/x86_64-linux-gnu PATH=/tmp/usr/bin:$PATH go run ratt.go /tmp/jq_1.5-1_amd64.changes
2015/10/12 20:49:06 Loading changes file "/tmp/jq_1.5-1_amd64.changes"
2015/10/12 20:49:06  - 1 binary packages: jq
2015/10/12 20:49:06  - corresponding .debs (will be injected when building):
2015/10/12 20:49:06     jq_1.5-1_amd64.deb
2015/10/12 20:49:08 a-g indextargets
2015/10/12 20:49:08 sourcesPaths = [/var/lib/apt/lists/ftp.ch.debian.org_debian_dists_unstable_main_source_Sources /var/lib/apt/lists/ftp.ch.debian.org_debian_dists_unstable_contrib_source_Sources /var/lib/apt/lists/ftp.ch.debian.org_debian_dists_unstable_non-free_source_Sources]

midna ~/gocode/src/github.com/Debian/ratt master 2 $ go run ratt.go /tmp/jq_1.5-1_amd64.changes
2015/10/12 20:49:15 Loading changes file "/tmp/jq_1.5-1_amd64.changes"
2015/10/12 20:49:15  - 1 binary packages: jq
2015/10/12 20:49:15  - corresponding .debs (will be injected when building):
2015/10/12 20:49:15     jq_1.5-1_amd64.deb
2015/10/12 20:49:15 fallback
2015/10/12 20:49:15 sourcesPaths = [/var/lib/apt/lists/ftp.ch.debian.org_debian_dists_unstable_contrib_source_Sources /var/lib/apt/lists/ftp.ch.debian.org_debian_dists_unstable_main_source_Sources /var/lib/apt/lists/ftp.ch.debian.org_debian_dists_unstable_non-free_source_Sources]

Debian/ratt