"no buildable Go source files"

Given this directory structure:

a $ tree
.
├── a.go
└── b
    ├── b.go
    └── c
        └── c.go

2 directories, 3 files

a $ cat a.go
package main

import (
    "a/b"
    "a/b/c"
)

func main() {
    b.B()
    c.C()
}

a $ cat b/b.go
package b

func B() {
    println("B")
}

 a $ cat b/c/c.go
package c

func C() {
    _ = "breakpoint"
    println("C")
}

Running godebug without -instrument gives a helpful warning:

 a $ godebug run a.go
godebug run: Ignoring breakpoint at a/b/c/c.go:4 because package "c" has not been flagged for instrumentation. See 'godebug help run'.

B
C

But the -instrument flag does not work as expected:

$ godebug run -instrument a/b/c a.go
/var/folders/4l/d1lsmz5d4972rfkg1gvv8hmc0000gn/T/godebug895759339/a.go:4:2: no buildable Go source files in /var/folders/4l/d1lsmz5d4972rfkg1gvv8hmc0000gn/T/godebug895759339/src/a/b

Start with the sample package tree and reproduce godebug run -instrument a/b/c a.go, focusing on how the -instrument package path is resolved for imports from a.go. Compare this with the warning from running without -instrument; done means the command instruments a/b/c and runs the example without the "no buildable Go source files" error.