This is necessary to avoid a circular dependency in the loader (which
soon will need to read the Go version) and because it seems like a
better place anyway.

This is needed to make it available to more packages, for caching
purposes.

For caching, the version itself may not be enough during development.
But for regular releases, the version provides some protection against
accidentally using a cache entry that is invalid in a newer version.

It was broken for quite some time without anybody noticing...

This commit changes the way that packages are looked up. Instead of
working around the loader package by modifying the GOROOT variable for
specific packages, create a new GOROOT using symlinks. This GOROOT is
cached for the specified configuration (Go version, underlying GOROOT
path, TinyGo path, whether to override the syscall package).

This will also enable go module support in the future.

Windows is a bit harder to support, because it only allows the creation
of symlinks when developer mode is enabled. This is worked around by
using symlinks and if that fails, using directory junctions or hardlinks
instead. This should work in the vast majority of cases. The only case
it doesn't work, is if developer mode is disabled and TinyGo, the Go
toolchain, and the cache directory are not all on the same filesystem.
If this is a problem, it is still possible to improve the code by using
file copies instead.

As a side effect, this also makes diagnostics use a relative file path
only when the file is not in GOROOT or in TINYGOROOT.

This commit replaces the existing ad-hoc package loader with a package
loader that uses the x/tools/go/packages package to find all
to-be-loaded packages.