Fix XSS in go-get handling

[thockin]

Followup to #8694 - fix the tests.

Don't REQUIRE at least 1 character (which broke tests).

Retain trailing / on URLs if specified, but with simpler regex handling.

@ameukam @BenTheElder @cji

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[BenTheElder]

Don't REQUIRE at least 1 character (which broke tests).

I want to say requiring a character in this particular match was necessary but it's been a while, maybe we should be matching the no-characters case seperately? (and that case shouldn't allow query params etc?)

[thockin]

I didn't want to go refactoring all the config, so I opted to make it as similar in function as possible. I agree that it's a little weird to have the same location handling all of those cases...