Local lib/ libraries are compiled regardless of the sketch's include graph

[stevenmunn312-tech]

A local library that no include reaches is still compiled, so one platform-specific
library sitting in lib/ breaks builds for every other platform.

Reproduce

A project whose lib/ holds FastLED and Adafruit_ZeroDMA (a SAMD library), and a
sketch that includes only <FastLED.h>:

$ fbuild build -e arduino_avr_uno --no-timestamp
build error: build failed: local library 'Adafruit_ZeroDMA' compilation failed:
build failed: compilation failed for .\lib\Adafruit_ZeroDMA\Adafruit_ZeroDMA.cpp:
lib/Adafruit_ZeroDMA/Adafruit_ZeroDMA.cpp:30:10: fatal error: malloc.h: No such file or directory
 #include <malloc.h> // memalign() function
          ^~~~~~~~~~
compilation terminated.

Nothing in the sketch includes Adafruit_ZeroDMA. It is being compiled for an ATmega328P
because it happens to sit in lib/.

Expected

Compile the libraries the include graph reaches, as #205's LDF-style scanner does for
framework libraries. Local lib/ directories appear not to go through it.

Workaround we run

Every optional library is renamed out of lib/ for the duration of a build and back
afterwards, keyed on the include markers the sketch actually contains. It works, but two
things follow from it: concurrent builds can never share a project directory, and we
cannot move those libraries to lib_deps — which resolves correctly as of 2.5.21, thank
you — because they would be back in the same position.

Environment

fbuild 2.5.21, Windows 11. Found while re-verifying our fbuild integration in
Design Studio for FastLED.