golang: Ensure to case-encode URLs for retrieving package info

When a go package name contains uppercase characters in it, associated goproxy URLs need to be case-encoded by replacing every uppercase letter with an exclamation mark followed by the corresponding lower-case letter.

This fixes the loading of such packages.

See https://go.dev/ref/mod#goproxy-protocol.

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Migrated from D8496 (view on Phabricator)