The introduction of `use <file.nu>` added the possibility of calling
`working_set.add_file()` more than once per parse pass. Some of the
logic handling the file contents offsets prevented it from working and
hopefully, this commit fixes it.
Currently, `use spam.nu` creates a module `spam`. Therefore, after the
first `use`, it is possible to call both `use spam.nu` and `use spam`
with the same effect.
In some rare cases, the global predeclarations would clash, for example:
> module spam { export def foo [] { "foo" } }; def foo [] { "bar" }
In the example, the `foo [] { "bar" }` would get predeclared first, then
the predeclaration would be overwritten and consumed by `foo [] {"foo"}`
inside the module, then when parsing the actual `foo [] { "bar" }`, it
would not find its predeclaration.
* Fixes a panic with defining two commands with the same name caused by
declaration not found after predeclaration.
* Adds a new error if a custom command is defined more than once in one
block.
* Add some tests
* Hiding logic is simplified and fixed so you can hide and unhide the
same def repeatedly.
* Separates predeclared ids into its own data structure to protect them
from hiding. Otherwise, you could hide the predeclared variable and
the actual def would panic.
