xbps-repo: made 'genindex' code run ~60% faster than before!

Rather than externalizing the package index plist file to storage every
time a package is registered, only do it one time once all packages
have been processed. Creating an index with 700 pkgs now takes 14s
in a VM, where before it took 39s. As consequence of this, the 'add-pkgidx'
target is no longer relevant or useful, remove it.

--HG--
extra : convert_revision : xtraeme%40gmail.com-20091118053439-ct1chn4hdeir1pdo

bin/xbps-repo/index.h

@@ -27,6 +27,6 @@
 #define _XBPS_REPO_INDEX_H_
 
 int	xbps_repo_genindex(const char *);
-int	xbps_repo_addpkg_index(const char *, const char *);
+int	xbps_repo_addpkg_index(prop_dictionary_t, const char *);
 
 #endif /* !_XBPS_REPO_INDEX_H_ */