Release BUCK files for glean itself

[lf-]

Hi,

To preface, we're a bit weird and willing to do tricky adaptation work. At my work we use buck2, and we've found the number of hacks necessary to build glean with nix to be a bit cumbersome, and cabal builds are rather slow. It would probably be helpful to have access to the buck2 definitions for Glean to potentially be able to build it directly with buck2 instead.

I'm not fussed about having access to the bzl bits outside //glean to make it work out of the box, since that's likely to be a bit of work to have to shim in a similar fashion to how buck2 itself has a shim directory to deal with the fbcode names for things. But having the BUCK files to work with would make our dev on glean easier.