Different source orders can affect output

The following targets give different output (aside from file name):

load("//mojo:mojo_library.bzl", "mojo_library")

mojo_library(
    name = "lib1",
    srcs = [
        "__init__.mojo",
        "foo/__init__.mojo",
    ],
)

mojo_library(
    name = "lib2",
    srcs = [
        # do not sort
        "foo/__init__.mojo",
        "__init__.mojo",
    ],
)

We seem to be relying on the fact that srcs[0] is in the root directory, so if the first source is not in the root directory then we only compile a subset of the files.

We could look for the root-most __init__.mojo but I would need to be certain that we can rely on that assumption.

In any case, generally we don't run into this since __init__.mojo always gets sorted to the first item by glob and by buildifier (if specified manually).

Start with the mojo_library rule loaded from //mojo:mojo_library.bzl and reproduce the issue using the two source orders shown. Trace how srcs[0] determines compilation, then verify that both orders produce the same output and compile all listed files.