option Prefix for needsPackage and friends instead of FileName

I wave my fist at the sky every time I have to laboriously type
```m2
needsPackage("PkgName", FileName => ".../PkgName.m2")
```
I mean, the whole `/PkgName.m2` part is unnecessary (not to mention File**N**ame ...)

Solution: add option `Prefix` that accepts a directory and deprecate `FileName` (i.e. still accept it but maybe don't mention it in the documentation).

The issue names needsPackage and related functions but no files or tests; start by locating their implementation and tracing how FileName is parsed and resolved. Done means Prefix accepts a directory, existing FileName usage remains accepted but deprecated, and the resulting package lookup behavior is verified.

Written by the indexing model from the issue text.