Mk/Uses/go.mk: Escape module names

Escape module names according to the GOPROXY protocol [1].

"To avoid ambiguity when serving from case-insensitive file systems,
the $module and $version elements are case-encoded by replacing every
uppercase letter with an exclamation mark followed by the corresponding
lower-case letter."

[1] https://golang.org/ref/mod#module-proxy

Reported by: yuri