Adds archive type detection and zip support to ExtractArchiveFile().

[tiegz]

Fixes #971

• internal/archive_extract.go

  • adds detectFileType() to detect archive file type (using mime sniffing)
  • adds processZipFile() to extract a .zip file

• internal/archive_extract_test.go

  • adds createZipFile helper to create a zip file fixture
  • adds doZipExtractionTest helper to run zip assertions
  • adds TestExtractSimpleZipFile test
  • adds TestExtractAbsolutePathZipFile test

Notes:

• I'm on a Mac so I'm unable to do an end-to-end test
• PyPI and NPM are the only package managers with extractArchive set, but AFAICT NPM only serves tarballs, so PyPI is the only package manager that would use this

[tiegz]

moved this out of extractTar() so it runs earlier