Fix namespace collision in std and nupm (#14009)

# Description

Fxes https://github.com/nushell/nupm/issues/102

Not loading `std` at startup has caused an issue with nupm and std where
the `dirs` module name clashes and nupm won't load. This was technically
a preexisting bug that was previously masked. This could have been fixed
(and also should be) by changing the import statement in nupm, but the
potential for collision would remain in other (user) modules.

This PR explicitly sets the relative path for the import statements in
`std/mod.nu` so that there are no collisions.

# User-Facing Changes

Bugfix

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

N/A

crates/nu-std/std/mod.nu

@@ -4,21 +4,21 @@
 export use lib *
 
 # std submodules
-export module assert
+export module ./assert
-export module bench
+export module ./bench
-export module dt
+export module ./dt
-export module formats
+export module ./formats
-export module help
+export module ./help
-export module input
+export module ./input
-export module iter
+export module ./iter
-export module log
+export module ./log
-export module math
+export module ./math
-export module xml
+export module ./xml
 
 # Load main dirs command and all subcommands
-export use dirs main
+export use ./dirs main
-export module dirs {
+export module ./dirs {
-  export use dirs [
+  export use ./dirs [
     add
     drop
     next