This reverts commit ab018c76e1.
This also adds the `ToolBuild::is_ext_tool` field to replace the previous
`ToolBuild::expectation` field, to indicate whether a build-failure of
certain tool is essential.
This commit enables configuring the RLS/rustfmt tools to the "broken" state and
actually get it past CI. The main changes here were to update all dist-related
code to handle the situation where the RLS isn't available. This in turn
involved a homegrown preprocessor-like-function to edit the configuration files
we pass to the various combined installer tools.
