Move avg and impvisitor examples to test/dub_package

[marler8997]

With this change the avg and impvisitor examples are now tested by ci.sh instead of from the Makefiles. This is a result of a suggestion from @wilzbach (#10504 (comment))

Note that I modified ci.sh to only build impvisitor.d instead of also running it. This is because it fails to run because of:

/home/runner/dmd/test/dub_package/../../../phobos/std/typetuple.d(42): Error: Outside Unicode code space

However, we currently aren't running impvisitor anyway.

[wilzbach]

Sorry, but as the last PR was merged so quickly without a discussion I think I have to be harsher here.
As explained on other PRs, the entire DMD as a library build was a PoC and is not used by a single person as there's dub now. The idea of a new build script was to remove the old clutter not aggregate it.
The examples can be moved into test/dub_examples.

[marler8997]

@wilzbach I'm confused, this PR is moving the examples to test/dub_packages like you asked. Why did you close it?

[dlang-bot]

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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#10552"

[wilzbach]

I'm sorry, I was too fast here.

[MoonlightSentinel]

Maybe remove the corresponding TODOs from build.d:

dmd/src/build.d

Lines 552 to 558 in 65d4a7c

	case "check-examples":
	"TODO: cxx-unittest".writeln; // TODO
	break;
	case "build-examples":
	"TODO: build-examples".writeln; // TODO
	break;