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.
This commit adds a new flag to the configure script,
`--enable-extended`, which is intended for specifying a desire to
compile the full suite of Rust tools such as Cargo, the RLS, etc. This
is also an indication that the build system should create combined
installers such as the pkg/exe/msi artifacts.
Currently the `--enable-extended` flag just indicates that combined
installers should be built, and Cargo is itself not compiled just yet
but rather only downloaded from its location. The intention here is to
quickly get to feature parity with the current release process and then
we can start improving it afterwards.
All new files in this PR inside `src/etc/installer` are copied from the
rust-packaging repository.
