packagefs: Package: Add versionedName property
packagefs: Add automatic SYS:PACKAGE_FILE attribute

* Rename SYS:PACKAGE attribute to SYS:PACKAGE_FILE.
* Re-add SYS:PACKAGE attribute. It contains the versioned name of the
  package.
BeBuild.h: Fix B_DEFINE_WEAK_ALIAS for use in C++ code
image.h: Add B_{APP,CURRENT}_IMAGE_SYMBOL macros

* Those can be used in cases where a pointer to an image symbol is
  required.
* Adjust BResources::SetToImage() accordingly.
Add new API find_path[s](), find_path_for_path()

The new functions are meant to replace many uses of find_directory():
* find_paths() is supposed to be used when the directories of a certain
  kind in all installation directories are needed (e.g. font
  directories, add-on directory, etc.). Using this API makes code
  robust wrt addition or removal of installation locations.
* find_path() is supposed to be used when files/directories associated
  with a loaded program, library, or add-on need to be found (e.g. data
  files or global settings).
* find_path_for_path() is similar to find_path(), but it starts from a
  given path instead of an image.
Add class BPathFinder

It's a more convenient C++ wrapper for the new find_path*() C functions.
Add findpaths command line tool

It provides the new find_path*() functionality to the shell.