For example, when running the following:
crates/nu-cli/src
nushell currently parses this as an external command. Before running the command, we check to see if
it's a directory. If it is, we "auto cd" into that directory, otherwise we go through normal
external processing.
If we put a trailing slash on it though, shells typically interpret that as "user is explicitly
referencing directory". So
crates/nu-cli/src/
should not be interpreted as "run an external command". We intercept a trailing slash in the head
position of a command in a pipeline as such, and inject a `cd` internal command.
This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).
