Add source mapping to codegen'd LLB

[hinshun]

• Source maps runtime LLB errors back to HLB (incl. a stacktrace)

 => ERROR copy /foo /                                                                                                                                                 0.0s
------
 > copy /foo /:
------
foo.hlb:2
--------------------
   1 |     fs func1() {
   2 | >>> 	func2
   3 |     }
   4 |
--------------------
foo.hlb:6
--------------------
   4 |
   5 |     fs func2() {
   6 | >>> 	func3
   7 |     }
   8 |
--------------------
foo.hlb:11
--------------------
   9 |     fs func3(){
  10 |     	scratch
  11 | >>> 	copy fs {
  12 |     		image "alpine"
  13 |     	} "/foo" "/"
--------------------
failed to solve: rpc error: code = Unknown desc = failed to compute cache key: "/foo" not found: not found

TODO

• Waiting for BuildKit PR to merge: llb: add source tracking support moby/buildkit#1494
• Need to fix source mapping for relative and remote imports
• Waiting for side effects PR to merge: Side effects and Binds #164

[coryb]

We should probably add an ErrOutput os.Writer to the RunOptions so we can use that here and plumb it through from newt.