Tools: Resolve virtual package requirements

At the moment, we require the virtual packages libpng3-dev and
libxslt-dev.  This leads to Puppet spamming on every run:

| notice: /Stage[main]/Toollabs::Dev_environ/Package[libxslt-dev]/ensure: ensure changed 'purged' to 'present'
| notice: /Stage[main]/Toollabs::Dev_environ/Package[libpng3-dev]/ensure: ensure changed 'purged' to 'present'

Also, it is not determinate which package is chosen to satisfy the
virtual requirement.  To solve this, we explicitly state which "real"
packages we require.

Change-Id: Ic0ce42aa96232e26a30b0c652e6ea1920d2e4322