Improve handling of package dependencies, support CMake and Meson

[elprans]

This is a fairly large-looking refactor aimed to replace manual
dependency configuration in package classes with automatic discovery
using pkg-config or CMake-provided package configs. Most packages ship
those and setting PKG_CONFIG_PATH and CMAKE_PREFIX_PATH is far more
robust than fiddling with *FLAGS directly.

This also adds full support for the CMake and Meson build systems and
makes lots of small changes required to do so in a clean fashion and
also to reduce boilerplate in package classes.

Specifically:

• overloading get_configure_script is no longer necessary for packages
  using supported build systems, get_configure_args and
  get_configure_env should be overloaded instead, returning a clean
  dictionary of flags instead;

• related to above, calling configure_dependency directly is not
  required anymore, dependencies are auto-configured based on their
  own build system declaration;

• get_build_script and get_build_install_script overloads should not
  call make (or whatever build system equivalent) and should instead
  call get_build_command() and get_build_install_command()
  correspondingly which take care of command formatting and injection of
  environment and global arguments.

Additionally, the root package can now control the overall layout of the
package bundle:

• get_dep_install_subdir may be overloaded to put all dependencies under
  a specified root, which is useful for packaging plugins and extensions
  that have dynamic library dependencies which should ideally be isolated
  with from similar dependencies of the host program;

• get_root_install_subdir may be overloaded to customize the name of
  the common parent directory in the installation prefix, e.g in
  /opt/<subdir> or /usr/lib/<host-triple>/<subdir>.

Other misc improvements and fixes:

• add support for building with Ninja;
• allow packages to have multiple source tarballs;
• use {mandir} and {docdir} install patterns instead of hardcoded
  {datadir}-based paths;