Fix .gcda to .pico (bsd.lib.mk) and .o (bsd.prog.mk) dependency mapping

The logic wasn't using a filtered version of SRCS -- it was using the
unfiltered version. This was causing some hilarity with unnecessary
dependencies, like headers and the like in lib/libc/... .

Also use .pico with bsd.lib.mk, not .o -- .o doesn't use SHARED_C*FLAGS,
which doesn't contain _COV_FLAG, which means that the .gcda's would have
relied on objects that didn't build them as a side-effect.