I'm assuming you have a code which uses some packages compiled into Bitlib library & now you're searching for this library.
A quick google search led me to a book on a university page which says this - "Many of the examples in Chapters 2 through 7 use a library called Bitlib. In order to compile these examples, you must first compile the Bitpack.vhd source file and then create the Bitlib"
So on the same website i found this file - Bitpack.vhd (http://www.ece.uah.edu/~gaede/cpe422/book_vhdl/Bit_pack.vhd) which you must compile into a new library Bitlib.
Would have been much easier if you'd just said in your post where you downloaded your vhd files from.