aptdistro.py: fix parsing bug on 'Pre-Depends: '

The old parser failed in the presence of 'Pre-Depends: ' lines, such as:

    Package: libsdformat1
    Source: sdformat
    Version: 1.4.11-1~precise
    Architecture: amd64
    Maintainer: Jose Luis Rivero <jrivero@osrfoundation.org>
    Installed-Size: 1153
    Pre-Depends: multiarch-support
    Depends: libboost-filesystem1.46.1 (>= 1.46.1-1), libboost-regex1.46.1 (>= 1.46.1-1), libboost-system1.46.1 (>= 1.46.1-1), libc6 (>= 2.14), libgcc1 (>= 1:4.1.1), libstdc++6 (>= 4.6), libtinyxml2.6.2
    Conflicts: libsdformat1-nightly, libsdformat1-prerelease, sdformat, sdformat-nightly, sdformat-prerelease
    Provides: libsdformat1-provider
    Multi-Arch: same
    Homepage: http://sdformat.org
    Priority: extra
    Section: libs
    Filename: pool/main/s/sdformat/libsdformat1_1.4.11-1~precise_amd64.deb
    Size: 357628
    SHA256: ee95fb583dc3af8a41d3234a95a9e32a15e9fb116000c7444198c5eeef9ac835
    SHA1: 56035b6660b30484f12d38a55d55995382704d29
    MD5sum: bcff7e2c0a12e445fc4dae0f37ddb608
    Description: Simulation Description Format (SDF) parser - Shared library
     SDF is an XML file format that describes environments, objects, and robots
     in a manner suitable for robotic applications. SDF is capable of representing
     and describing different physic engines, lighting properties, terrain, static
     or dynamic objects, and articulated robots with various sensors, and acutators.
     The format of SDF is also described by XML, which facilitates updates and
     allows conversion from previous versions. A parser is also contained within
     this package that reads SDF files and returns a C++ interface.

Entries such as this lead to the following error message:

    "Found 'depends' but not 'package' while parsing the apt repository index file"

Enforcing that 'Depends: ' starts the line fixes that. This commit also 
enforces the same thing for 'Package: ' lines.