Skip dependency detection if library is fully precompiled (arduino#1139)

* Skip dependency detection if library is fully precompiled

Precompiled bits of a library should not depend on any link time dependency (we cannot assure ABI stability).
Fixes arduino/ArduinoCore-mbed#119

* Add output when skipping deps detection for precompiled libs

Co-authored-by: Silvano Cerza <silvanocerza@gmail.com>