Hadrian and haddock CI improvements

1. Depend on docs target before running tests in hadrian (#19072 (closed), #17728 (closed), #20176 (closed)) if we are going to be running any haddock tests
2. Build validate-x86_64-linux-deb9-debug with hyperlinked source (#20067 (closed))
3. Add a --test-have-intree-files flag to the hadrian test runner so that it can run in_compiler_tree tests even when testing the bindist

We teach the testsuite driver to report any extra hadrian targets it might need, like haddock docs. The driver only reports these targets if the tests in question are actually going to be run. In particular, if you pass --only="T1 T2 T3", and neither of T1,T2, or T3 require a hadrian target, then that target will not be built.

Then we add "docs-haddock" as a required target for all the haddock tests, taking care of point 1.

Fixes #19072 (closed), #17728 (closed), #20176 (closed), #20067 (closed), #20427 (closed), #19614 (closed), #16987 (closed)