Add Rich data for GitLab.com as code provider

[ZJvandeWeg]

When packages were searched for and displayed, platforms like GitHub and
Bitbucket had slightly better data presentation. For example the number
of stars a project has.

Additionally, the fallback Git provider that was being used for GitLab,
leverages git ls-remote and git fetch to determine what blobs are in
a tree, and what subdirectories. This combination of requests is much
slower than the 3 API calls done now.

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[ZJvandeWeg]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[gopherbot]

This PR (HEAD: 11db952) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/gddo/+/205417 to see it.

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[gopherbot]

Message from Gobot Gobot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
Within the next week or so, a maintainer will review your change and provide
feedback. See https://golang.org/doc/contribute.html#review for more info and
tips to get your patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
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---

Please don’t reply on this GitHub thread. Visit golang.org/cl/205417.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Go Bot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
Within the next week or so, a maintainer will review your change and provide
feedback. See https://golang.org/doc/contribute.html#review for more info and
tips to get your patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11, it means that this CL will be reviewed as part of the next development
cycle. See https://golang.org/s/release for more details.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/205417.
After addressing review feedback, remember to publish your drafts!