More linker modeling

[woodruffw]

As part of doing a better job of tracking inputs and outputs, we should model the following in the linker (LD) and all compiler tools:

• -L$path: prepend $path to the library search path
• -l$name: search for $libname in the library search path

In particular, it's common for builds to produce intermediate libraries and then link them into their final "leaf" binaries. When this happens, the command line usually looks something like this:

$CC -o foo foo.c -Llocal/path -lutils

In cases like that, we want to opportunistically resolve -lutils as local/path/libutils.{so,a} if that path is actually present.