fetchMavenArtifact downloads a Maven artifact given a group id, an artifact id,
and a version.
Example usage:
org_apache_httpcomponents_httpclient_4_5_2 = fetchMavenArtifact {
groupId = "org.apache.httpcomponents";
artifactId = "httpclient";
version = "4.5.2";
sha256 = "0ms00zc28pwqk83nwwbafhq6p8zci9mrjzbqalpn6v0d80hwdzqd";
# Optionally: repos = [ ... urls to some Maven repos to use ... ];
# Optionally: url, urls - pointing directly to a specific jar url.
};
Now `org_apache_httpcomponents_httpclient_4_5_2.jar` points to the downloaded
JAR file, while `org_apache_httpcomponents_httpclient_4_5_2` refers to a
derivation that when used used in `buildInputs` will be automatically added to
the Java classpath.
