pruneLibtoolFiles: init setup hook (#41819)

A .la file specifies linker flags to link with the library it describes. Its
"dependency_libs" field lists the libraries that this library depends upon.
This list often contains "-l" flags without corresponding "-L" flags. Many
packages in Nixpkgs deal with this in one of these ways:
- delete .la file [1]
- clear dependency_libs [2]
- add -L flags to dependency_libs [3]
- propagate dependencies [4]

Sometimes "dependency_libs" contain wrong "-L" flags pointing to the "dev"
output with headers rather than to the main output with libraries. They have to
be edited or deleted to reduce closure size [5].

Deleting .la files is often but not always safe [6].  Atomatically deleting as
many of them as possible is complex [7].  Deleting .la files that describe
shared rather than static libraries is probably safe; but clearing their
"dependency_libs" field achieves the same effect with less potential for
unintended consequences.  This is the approach that may be enabled for all
Nixpkgs.

[1] https://github.com/NixOS/nixpkgs/commit/2a79d296d3de74da77b49ca8dc9d356b1a7a1c8d
[2] https://github.com/NixOS/nixpkgs/commit/c83a53098599c49ec57823bf655ce8b45f6aea10
[3] https://github.com/NixOS/nixpkgs/commit/9e0dcf3bd9fcfeed576132b9101b866aa3cb76ff
[4] https://github.com/NixOS/nixpkgs/commit/01134e698fdfb3b61f8ab8ec3bf13e0718f63955
[5] https://github.com/NixOS/nixpkgs/commit/f6c73f1e37c735abb686d07449f349b796c54cf8
[6] https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Handling_Libtool_Archives
[7] https://github.com/gentoo/gentoo/blob/fb1f2435/eclass/ltprune.eclass