collect module imports of public modules

[iceghost]

Currently, zls only collects module imports of Step.Compile. For public modules, it only collect the root_modules, not their module imports.

This commit makes the build_runner also collects the module imports and also adds one regression test case when one does addOptions on a public module, which is my original motivation (and confusion why my @import("options") was not working).

The generated options.zig file is generated in the c/ instead of o/ namespace, so I also removed the namespace assertion. I assume the original assertion is only for reminding reader of what that component is, and not an actual useful property of the path, so I keep the intent as a comment.

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Please let me know if this is the correct fix, or was there a reason b.modules are treated differently from Step.Compile.

Also, without the fix, the added test case with fail with:

getPath() was called on a GeneratedFile that wasn't built yet.
  source package path: /workspace/zls/tests/build_runner_cases
  Is there a missing Step dependency on step 'options'?

[Techatrix]

👍