Windows: enable "spack install" tests (#34696)

* The module-level skip for tests in `cmd.install` on Windows is removed.
  A few classes of errors still persist:

  * Cdash tests are not working on Windows
  * Tests for failed installs are also not working (this will require
    investigating bugs in output redirection)
  * Environments are not yet supported on Windows

  overall though, this enables testing of most basic uses of "spack install"
* Git repositories cached for version lookups were using a layout that
  mimicked the URL as much as possible. This was useful for listing the
  cache directory and understanding what was present at a glance, but
  the paths were overly long on Windows. On all systems, the layout is
  now a single directory based on a hash of the Git URL and is shortened
  (which ensures a consistent and acceptable length, and also avoids
  special characters).
  * In particular, this removes util.url.parse_git_url and its associated
    test, which were used exclusively for generating the git cache layout
* Bootstrapping is now enabled for unit tests on Windows

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