Accurately list htslib/samtools/bcftools contents in MANIFEST.in
Accurately list htslib/samtools/bcftools contents in MANIFEST.in

Rewrite these inclusions so that the sdist tarball contains everything
checked into the repository. In particular:

* Add new htscodecs source files and build scripts
* Add LICENSE files and READMEs
* Exclude new config_vars.h and samtools_config_vars.h generated headers

pysam-developers/pysam,pysam-developers/pysam,pysam-developers/pysam,pysam-developers/pysam