module paths can resolve incorrectly

[radarroark]

Zig Version

0.11.0-dev.3132+465272921

Steps to Reproduce and Observed Behavior

This bug already has a PR at #15717, just adding an issue for it as well.

To reproduce, create a zig project with another zig project in a subdirectory:

mkdir outer && cd outer
zig init-exe
mkdir inner && cd inner
zig init-exe
mv src/main.zig src/inner.zig

Now the outer project looks like this:

├── build.zig
├── inner
│   ├── build.zig
│   └── src
│       └── inner.zig
└── src
    └── main.zig

Now open inner/build.zig and make these changes so it points to the outer project:

diff --git a/inner/build.zig b/inner/build.zig
index d6b4a01..e3b823a 100644
--- a/inner/build.zig
+++ b/inner/build.zig
@@ -19,10 +19,13 @@ pub fn build(b: *std.Build) void {
         .name = "inner",
         // In this case the main source file is merely a path, however, in more
         // complicated build scripts, this could be a generated file.
-        .root_source_file = .{ .path = "src/main.zig" },
+        .root_source_file = .{ .path = "src/inner.zig" },
         .target = target,
         .optimize = optimize,
     });
+    exe.addAnonymousModule("outer", .{
+        .source_file = .{ .path = "../src/main.zig" },
+    });

Finally, in outer, run this:

zig build --build-file inner/build.zig

You will see:

zig build-exe inner Debug native: error: warning: FileNotFound: inner/src/main.zig
error: FileNotFound

No bueno!

Expected Behavior

It should print wicked cool centaur ANSI art with an inspirational quote. Just kidding. Compilation should succeed because the path in inner/build.zig is ../src/main.zig, so it should resolve to src/main.zig, not to inner/src/main.zig.

[radarroark]

@Vexu may I have a nice and shiny build system tag?