Run pkg/pub_dartdoc instead of global dartdoc

[isoos]

#1179

I'll deploy this to staging with the initialization PR to test it.

[isoos]

Staging looks good, pub-data.json is generated as expected:
https://dartdoc-dot-dartlang-pub-dev.appspot.com/documentation/pana/latest/pub-data.json

[isoos]

This is not flawless in staging, e.g. sometimes we get this warning from dartdoc and it breaks the further processing:
stderr: warning: dartdoc detected an unknown file in the doc tree: pub-data.json

[isoos]

Looks like that warning does not affect the results, only it gets more visible when there are no .dart files in the package, but the log warning is about not having files there, the warning is unrelated.
As there is no quick way to fix the warning, and otherwise it works, I'm merging this.

[isoos]

Oh, merging... after it gets reviewed :)

[sortie]

lgtm

Whoops forgot to submit my review.

[sortie]

What about other error codes besides 0 and 15?

[isoos]

15 is SIGTERM, which we get when the timeout kills the process. In that case we retry it without link validation, which is a time-consuming thing to do on a large archive. On any non-zero we emit the the log.warning about it.

However we don't really care about the exit code besides these two, as long as the run produced index.html and index.json (and we may add pub-data.json to that list too).