internal/gps: fix case mismatch error with multiple dependers

[glasser]

What does this do / why do we need it?

Fixes a bad error message added in dep 0.3.1.

Previously, errors like the following could occur:

v0.4.2: Could not introduce github.com/sirupsen/logrus due to a case-only variation: it depends on "github.com/Sirupsen/logrus", but "github.com/evalphobia/[email protected]" was already established as the case variant for that project root by the following other dependers:
(root)
github.com/docker/docker@8510adf8c856d6f0871650216a0e1d7e6ece46ee

This fix matches the correct behavior when there is only one depender earlier in
the function.

What should your reviewer look out for in this PR?

Is there an opportunity to add a test for this? I didn't see one.

Do you need help or clarification on anything?

No

Which issue(s) does this PR fix?

I didn't file an issue but I did jot down a comment on the PR that introduced this issue: #1079 (comment)

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[googlebot]

CLAs look good, thanks!

[glasser]

Resubmitted with the email address that has a CLA.

[sdboyer]

yes yes, thank you!