boost fetching plugin prefix

[Eskibear]

Currently we run mvn help:describe and parse its output to get information like prefix and goals. It can be slow to run commands for all plugins at the same time (and that's why we cache result).

Now I find prefix can be directly found from maven central.

E.g.
For all plugins with group id org.apache.maven.plugins, information like artifactId, name, prefix can be read directly from
https://repo1.maven.org/maven2/org/apache/maven/plugins/maven-metadata.xml

[Eskibear]

To verify:

1. Open a maven project
2. expand project node, expand "Plugins" node
3. it's expected to see plugins listed with groupid and artifact id at first
4. later the label should be replaced by its "prefix". And it should not a long time.