Introduce default-mysql-* virtual packages to be used in Build-depends

Current virtual-mysql-* scheme is not usable in Build-Depends, because
a Build-Depend must point to an exact package and not an option.
This scheme can also be used in other places where a strict default
package dependency is needed. For more lax scenarios where the decision
is left to the packager of other packages (who might have done some
manual testing and decisions based on that) so they can choose what
is best for their package dependencies.

This is modeled after how openjdk provides default-jdk as a purely
virtual package which is used as build dependency in e.g. ant and as a
normal dependency in many other packages.

Summary:
- virtual-mysql-* stays. Any MySQL variant is free to provide it.
- default-mysql-* is new. Only the package defined by the release team
  is allowed to provide it.

debian/control

@@ -87,7 +87,7 @@ Replaces: mariadb-client-core-5.5,
           mysql-client-core-5.6,
           mysql-client-core-5.7,
           virtual-mysql-client-core
-Provides: virtual-mysql-client-core
+Provides: virtual-mysql-client-core, default-mysql-client-core
 Description: MariaDB database core client binaries
  MariaDB is a fast, stable and true multi-user, multi-threaded SQL database
  server. SQL (Structured Query Language) is the most popular database query
@@ -121,7 +121,7 @@ Replaces: mariadb-client-5.5,
           mysql-client-5.6,
           mysql-client-5.7,
           virtual-mysql-client
-Provides: virtual-mysql-client
+Provides: virtual-mysql-client, default-mysql-client
 Description: MariaDB database client binaries
  MariaDB is a fast, stable and true multi-user, multi-threaded SQL database
  server. SQL (Structured Query Language) is the most popular database query
@@ -156,7 +156,7 @@ Replaces: mariadb-client-10.0 (<< ${source:Version}),
           mysql-server-core-5.6,
           mysql-server-core-5.7,
           virtual-mysql-server-core
-Provides: virtual-mysql-server-core
+Provides: virtual-mysql-server-core, default-mysql-server-core
 Description: MariaDB database core server files
  MariaDB is a fast, stable and true multi-user, multi-threaded SQL database
  server. SQL (Structured Query Language) is the most popular database query
@@ -186,7 +186,7 @@ Replaces: mariadb-test-10.0,
           mysql-testsuite-5.6,
           mysql-testsuite-5.7,
           virtual-mysql-testsuite
-Provides: virtual-mysql-testsuite
+Provides: virtual-mysql-testsuite, default-mysql-testsuite
 Suggests: patch
 Description: MariaDB database regression test suite
  MariaDB is a fast, stable and true multi-user, multi-threaded SQL database
@@ -262,7 +262,7 @@ Replaces: mariadb-galera-server,
           mysql-server-5.6,
           mysql-server-5.7,
           virtual-mysql-server
-Provides: virtual-mysql-server
+Provides: virtual-mysql-server, default-mysql-server
 Description: MariaDB database server binaries
  MariaDB is a fast, stable and true multi-user, multi-threaded SQL database
  server. SQL (Structured Query Language) is the most popular database query