buoy: cut extra URL path for module

[nak3]

Some module use sub-directory for the module like https://github.com/blang/semver/tree/master/v4

In that case, buoy fails to fetch metadata with the full path:

$ curl https://github.com/blang/semver/v4?go-get=1

so needs to use the top directory and path.

$ curl https://github.com/blang/semver?go-get=1

Hence, this patch cuts extra URL path for module.

[nak3]

Background issue is knative/pkg#2759 (comment)
I would like to use buoy to bump third-party library but it fails due to the issue.

[nak3]

/cc @kvmware

[knative-prow]

@nak3: GitHub didn't allow me to request PR reviews from the following users: kv.

Note that only knative members and repo collaborators can review this PR, and authors cannot review their own PRs.

In response to this:

/cc @kvmware @KV

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[krsna-m]

/lgtm
@cardil or @upodroid want to give it another pair of eyes and /approve it?

[krsna-m]

@nak3 note that the changes to hack you identified (in slack) will have to be done too if you want to pick up these changes

[upodroid]

LGTM

Don't forget to update knative/hack to use install bouy from this repo.

[knative-prow]

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