new package: py-youtube-dl + fixes for dependencies (#15978)

* new package: py-youtube-dl + fixes for dependencies

This PR adds the py-youtube-dl program. In addition, there are a couple
of dependency packages that needed to be updated.

* ffmpeg
This is needed by py-youtube-dl. However, the spack ffmpeg recipe does
not include a lot of options, specifically, a dependency on openssl for
working with the https protocol.

- Added updated version.
- Added variants for the different licensing options.
- Added "meta" variants for X and drawtext. These turn on/off several
  options.
- Set variants and dependencies for many options. The defaults are based
  on the configuration settings in ffmpeg.
- Set dependencies that were missing or that will likely get pulled in
  from the system.

* libxml2
The ffmpeg+libxml2 variant initially failed to build. The issue is that
libxml2 sets the headers property to

include_dir = self.spec.prefix.include.libxml2

The ffmpeg configure looks for prefix.include and fills in the rest.
This could probably be patched in ffmpeg but the headers property in the
libxml2 recipe is not consistent with the environment module or the
pkgconfig file, both of which set the headers path to prefix.include.
This PR sets the libxml2 headers property to

include_dir = self.spec.prefix.include

A spot check of a few libxml2 dependents did not rreveal any problems
with this change.

* Comment out libxml2 dependency in ffmpeg

The header property issue of the spack libxml2 package will need to be
resolved in another PR before libxml2 can be enabled in ffmpeg.

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