druntime docs generator: replaces src/* by import/*

[denizzzka]

Actually, we want to generate documentation for modules that user can import. Files from import/ are almost same files as in src/.

This may help us to avoid tricky errors related to documenting inaccessible for end users modules (as rt.llmath module, which is not copied into import/ and yet presented in mak/DOCS)

My motivation for this PR: this is need for #15822 - switchable OS-depended source files is too hard to use for docs generation.

The fact is that the paths of these modules are differ depending on the target system. And only import/ dir contains them always at same places because they are automatically copied into import/ as usual

(If #15822 will be accepted it opens way to generate sub-documents for each OS-specific module by a single pass)

Caution! PR changes site links!

-	<div class="tip smallprint">		<a href="https://github.com/dlang/druntime/edit/master/src/core/internal/atomic.d">Improve this page</a>
+	<div class="tip smallprint">		<a href="https://github.com/dlang/druntime/edit/master/import/core/internal/atomic.d">Improve this page</a>

Upd: https://github.com/dlang/druntime/ repository has been archived on Oct 12, 2022. It is now read-only. So I think this links changes isn't matter

Upd2: Or matter? This generated html is used somewhere, albeit with a different base site name?

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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 run digger -- build "master + dmd#15880"