This updates Bolt to support installing git-based modules from any
source. Now, Bolt will attempt to download metadata directly from GitHub
or GitLab for public repositories and fall back to cloning the git
repo's metadata if that fails.

This also updates the spec resolver to no longer calculate a SHA based
on the ref provided in project configuration. Previously, Bolt would
calculate a SHA and write it to the Puppetfile to pin the module to a
specific commit when modules are installed. However, doing this is
difficult and time-consuming with the move to supporting any git-based
module.

!feature

* **Support installing any git-based module**
  ([puppetlabs#3109](puppetlabs#3109))

  Bolt now supports installing any git-based module. Previously, Bolt
  only supported installing and resolving dependencies for modules
  hosted in a public GitHub repository. Now, Bolt will check both GitHub
  and GitLab before falling back to cloning a module's metadata using
  the `git` executable. This allows users to install modules from
  private repositories, or from locations other than GitHub.

This updates documentation for installing git modules to no longer say
only public GitHub repositories are supported.

This fixes a bug where the proxy configured in `module-install.proxy`
was not being set when retrieving metadata for git-based modules.
Previously, the proxy was only configured when resolving and
installing modules.

!bug

* **Configure proxy when retrieving metadata for git-based modules**

  Bolt now configures a proxy when `module-install.proxy` is set and it
  retrieves metadata for git-based modules. Previously, Bolt was only
  configuring a proxy when it resolved and installed modules.

This updates the `GitSpec` class to resolve the SHA for a git module's
ref.