Determine module isRoot earlier

[marler8997]

This PR is in preparation for other functionality (such as skipping unittests in imported modules). It modifies the code to determine if a module is a "root module" (a module being compiled to object code) as soon as the module name has been determined which is just after the module declaration has been parsed.

[dlang-bot]

Thanks for your pull request and interest in making D better, @marler8997! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the annotated coverage diff directly on GitHub with CodeCov's browser extension
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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#8151"

[marler8997]

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