Make use of fs.FS (new to go 1.16) to allow for reading source files from diverse filesystems (local, embed, custom).
* `Options` has a new field `SourcecodeFilesystem fs.FS` so users can supply their own read-only filesystem containing source code.
* Defaults to the local filesystems (via `RealFS` - a thin `os.Open` wrapper complying with `fs.FS`) so regular users should see no change in behaviour.
* When no filesystem is available (e.g. WASM, or if you want to embed files to retain single binary distribution) an alternative filesystem is preferable to using `Eval(string)` as that requires the stringy code to be a single file monolith instead of multiple files. By using an `fs.FS` we can use `EvalPath()` and gain the ability to handle multiple files and packages.
* You can make use of embed filesystems (https://pkg.go.dev/embed) and custom filesystems obeying the `fs.FS` interface (I use one for http served zip files when targeting wasm as there is no local filesystem on wasm). Tests can make use of `fstest.Map`.
* NOTE: This does NOT affect what the running yaegi code considers its local filesystem, this is only for the interpreter finding the source code.
See `example/fs/fs_test.go` for an example.
Fixes#1200.
b5bf4ef31a