* 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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