maven_install: fail if 'repositories' attribute is empty

[mattnworb]

Coursier fails with a cryptic error in case someone tries to use maven_install() without specifying any repositories, which is a small silly thing I did when trying to extract internal corporate code into a public proof-of-concept on how to do something with Bazel that took me 10 minutes to hunt down :)

When repositories is empty, Coursier fails with an error like:

  Dependencies:
org.junit.jupiter:junit-jupiter-engine:5.10.2:
Resolution error: Error downloading org.junit.jupiter:junit-jupiter-engine:5.10.2

this was for a maven_install() block with a single artifact to install; otherwise the error message will contain one Resolution error per artifact. The problem though is that Coursier doesn't say what the error is, just that one occurred.

The root-cause is that coursier_fetch etc will invoke coursier with arguments like

coursier fetch <artifacts> ... --no-default ...

which disables default repositories like Maven Central.

So as a simple fix to prevent people from configuring maven_install in a way that doesn't make sense like I did, this commit adds a check inside make_coursier_dep_tree (where the arguments to coursier are constructed) to fail if the list/dict of repositories is empty.

[jin]

I'm surprised that we took this long to encounter this failure mode... thanks!

Could you rebase?

[mattnworb]

@jin done, thanks!