Normalize submodule urls before looking at them

Allow adding a submodule through git_index_add_bypath

This also affects `git_index_add_bypath()` by providing a better error
message and a specific error code when a directory is passed.

When we rename a submodule, we should be merging two sets of information
based on whether their path is the same. We currently only deduplicate
on equal name, which causes us to double-report.

When two submodules are fairly similar, we may end up loading the wrong
one.

The function was removed, but its declaration and changelog entry about
its removal were forgotten.

The comment in the test doesn't make any sense as the function doesn't
exist anymore, so get rid of it as well.

We allow looking up a submodule by path, but we lost the path
normalisation during the recent changes. Bring it back.

Fallback describes the mechanism, while unspecified explains what the
user is thinking.

In addition to mapping enums to strings in the configuration, we need to
know to delete the configuration option when given the "none" or "no"
option.