fix(compat): an absolute DT_NEEDED is not a soname, and a missing tool is not an empty answer - #382

…l is not an empty answer

Two defects that only appear when `compat.glx-runtime`'s new pass is actually
executed. The previous change measured the opencl and vulkan farms; the glx one
had never been run.

AN ABSOLUTE `DT_NEEDED` NEVER GOES THROUGH A SEARCH PATH. Four members of this
farm -- the glvnd vendor entries `libEGL_nvidia`, `libGLESv1_CM_nvidia`,
`libGLESv2_nvidia` and `libGLX_nvidia` -- name `/lib/x86_64-linux-gnu/...`
outright. The loader opens those directly, so the farm can neither serve them
nor honestly record them as unserved. Treating one as a soname looks up a name
with slashes in it and then writes an `unserved` link whose NAME is a path. All
three recipes skip them.

A MISSING TOOL IS NOT AN EMPTY ANSWER. With `readelf` absent the pass returned
`{}`, which reads exactly like a farm with no gaps -- the confusion this whole
batch exists to remove, one layer down in the tool lookup. It now warns, and
`compat.glx-runtime`'s lookup reaches the xim store before PATH like the other
two, because this package runs on machines that were never asked to have a
toolchain.

Measured against the installed glx farm with the same logic: 52 members, 4
absolute entries skipped, 19 sonames needed, all 19 filled from ecosystem
payloads, 0 unserved, and no path-shaped entry.