SVN test safety and other improvements

A number of improvements from hakre. In particular, an important change with "safeguard prefix directory operations in SVN testing".

SemVer and Numeric Branches

• Handle semver and numeric branch names (constraints ending in "-dev") as refs. See https://getcomposer.org/doc/articles/versions.md#branches

Regression fix

• Fix joinPath() to handle absolute paths like it should
• Fix svn file name detection

Pluggable VCS with Subversion, Drupalcode fix

• Change vcs support to be pluggable
• Adds --vcs to force a specific vcs
• Adds subversion as vcs plug
• Fix drupalcode to use common gitlab format for compare
• Adds Development section to readme
• Remove @dev stability from composer package

Drupalcode support

• Added Drupal packages support
• Fix source reference access for dev- packages

PHP 7.3 compat, support bitbucket & gitlab, test data

• Removes flags FILTER_FLAG_SCHEME_REQUIRED | FILTER_FLAG_HOST_REQUIRED for PHP 7.3
• SSH format repos
• Test data

URLs and Filtering

• Handle URLs to composer.lock properly
• Add flags for filtering to dev or prod only

Compare links

Adds a Compare column with links to the diff at the source provider. Also adds the --no-links option to suppress the new behavior.

Paxal's dreams

Added better path handling and markdown table output. Ideas courtesy of @paxal.

Getopt any ref or file

Adds getopt options to be able to diff any composer lock from git ref or php readable stream.