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.