Fix a bug in the Hosted Git resolver, where it would make an HTTP request even if a mirrored tarball is present.

[chrisgavin]

Summary
After mirroring a package locally, specifically web-component-tester and attempting to install the dependency offline from the mirror I discovered Yarn makes an HTTP request. This means the installation fails if you pass --offline to Yarn or don't have an internet connection.

If in offline mode the error "Can't make a request in offline mode" is displayed, otherwise an error about being unable to resolve github.com is shown.

It appears this problem occurs because the package in question references a shorthand GitHub dependency ("test-fixture": "PolymerElements/test-fixture"). A URL for this is created which gets passed through the hosted Git resolver. The hosted Git resolver then attempts to determine if this URL "hasHTTPCapability" by making a request to it. This fails in offline mode.

The fix I have made here is simply to assert that the URL "hasHTTPCapability" if a mirror tarball for it exists. Sorry if this is not the right fix, but I am not very familiar with the codebase so I tried to make the simplest change possible.

Test plan

git clone https://github.com/chrisgavin/fake-package
cd fake-package
yarn install
yarn install --offline

This fails before applying my patch, but succeeds afterwards.

[chrisgavin]

I might also hesitantly suggest this resolves #1925.

[chrisgavin]

It seems I broke some tests. Closing this until I get a chance to fix them.

[TimvdLippe]

@chrisgavin The test failure on AppVeyor is also happening with other PRs, so I think it is unrelated to your PR 😄

[raymondwang]

@chrisgavin: This seems to work, and it's the singular feature preventing me from switching over to Yarn today. Can you reopen? I'd be happy to contribute unit tests if you don't have time as necessary.