Add ability to translate path mapping prefixes

Reads a list of path mapping prefixes and translations from a
'path-mapping-translations' section in composer.json's extras section. All
path mappings (from modman files, package.xml files or a composer.json map
section) are checked against these translations and changed accordingly. This
can be used to support versions of Magento that put certain types of files in
non-standard locations. For example, skin files might go under 'public/skin'
rather than just 'skin'.