We will need the Wasm backend to be built for 2.12.x in order to
load it as a true linker from the sbt build.

This way, we do not depend on Scala.js-specific APIs.

An `OutputDirectory` is not supposed to be shared.

And delete stray files in the output directories, as is mandated by
the contract of a linker backend.

This follows the architecture of the Scala.js linker.

In addition to emitting the `.wasm` and `.wat` files in the output
directory, we now also emit emit the internal `__loader.js` file,
and the public module specified by the `ModuleSet`, such as
`main.js`.

We embed the content of the loader as a constant string in the
backend, so that it works both on the JVM and on JS. Eventually, we
will probably generate it more dynamically from
`backend.javascript.Trees` anyway, so this not really a big deal.

This change requires to adapt the loading mechanism to force URL
resolution against the base directory of the `loader.js` file,
rather than the working directly. We do this thanks to
`import.meta.url`, which is also available in browsers.

As is, this change destroys the semantics of top-level exported
`var`s. Mutations performed after the module initializers have
completed will not be reflected in the exports of `main.js`. We
will need to revisit this later on, with a technique similar to the
one used by the JS backend.

With these changes, our linker backend actually abides by the
specification of a `LinkerBackend`, which means we will be able to
directly use it from sbt.

Now, `sample/fastLinkJS` uses our WebAssembly linker, and
`sample/run` actually executes the `main` method of the sample
project, linked to WebAssembly.