Previously, Maven repositories definition was specified in a profile that
is active by default. It means that as soon as any profile is enabled by
the user, said profile is no longer enabled. This has the nasty
consequences of having copy/paste in several places to make sure our own
profiles still have the proper repositories definition.

This commit creates a single "repositories" profile that is always active
unless a given property is explicitely specified. This allows to remove
the duplication and make things more consistent.

Some Gradle-specific repositories were also hard-coded in two modules
without any profile at all, meaning they were polluting the build of
anybody using it. While the impacted modules are gradle specific, that
repository has been shared in the new "repositories" profile as well.

Closes gh-6031