Deprecate MavenPomDownloader constructors

[knutwannheden]

The current MavenPomDownloader constructors are a bit confusing, especially since one of them is specific to Gradle and the others are intended to be used for Maven. (The difference being that in the Gradle case Maven Central and Maven Local should not be automatically added as repos.)

There are various ways to improve this. The simplest thing would be to make the constructors private and provide factory methods with "Maven" and "Gradle" in their names. There are probably other better options.

[shanman190]

Another option would be to have the Maven marker carry all repositories, including any default ones like Maven Central.