[R-package] always name the shared library 'lightgbm', not 'lib_lightgbm'

[jameslamb]

This proposes ensuring that the shared library for R packages builds is always called lightgbm.{dll,dylib,so}, not lib_lightgbm.{dll,dylib,so}.

• to help with this: [R-package] Use ALTREP system to return C++-allocated arrays #6213 (comment)
• to remove complexity that we don't need any more since the R package does not support rebuilding from a precompiled library ([R-package] Remove ability to install precompiled lib_lightgbm (fixes #3320) #3360)

Notes for Reviewers

Why not standardize on lib_lightgbm instead?

Much of the standard R tooling prefers that the main shared library for a package be named {package}.so, not lib_{package}.so. See, for example, this: https://cran.r-project.org/doc/manuals/R-exts.html#useDynLib.

That is why the package we ship to CRAN uses the library name lightgbm.{dll,dylib,so}.

Is this a breaking change?

No.

It does not change anything about the CRAN package.

For the CMake-based package, it changes the library filename but that library is only intended to be used by the {lightgbm} library, not externally linked to by any other programs.

Why did all this complexity exist in the first place?

Honestly.... because I didn't really understand CMake back when setting up this project's R packaging stuff a few years ago 😬

[borchero]

LGTM!