Get and stay current with third-party dependencies

[DHowett-MSFT]

• libxml2
• libz
• cassowary?
  • we have local patches to 0.60; they may not be portable
• icu
  • remove, depend on Windows inbox ICU (10.0.15063+)

[DHowett-MSFT]

2051 tracks switching to inbox ICU

[DHowett-MSFT]

libxml2 must be rebuilt with the new ICU