Create a utility class for scm url checking/parsing

There is a lot of code in different places, both in maven-scm and elsewhere, that checks and parses scm url:s. I propose that a utility class ScmUrlUtils be crated in maven-scm-api where such code (static methods) can be placed. The code there should not be scm provider specific. This should be accompanied by a test suite.

This concept might also be applied to individual scm providers, e.g. there could be a CvsScmUrlUtils in maven-scm-provider-cvs.