Disambiguate between libraries and .lib.* files

[jtigger]

The docs say...

• module: single file; can export variables, functions, or be templated => some type of result e.g. yaml structure, or string, or None)
• package: single directory; contains modules
• library: collection of packages

But the file extension that indicates that a file is a loadable module is .lib.(y[a]ml|star).

Using the same term for different concepts can lead to confusion. We could avoid that confusion if we disambiguate them.

[jtigger]

While the files we generally refer to as "templates" are technically implemented as Starlark modules, they are not loadable by the user.

How about this?

• template: single file; results in a corresponding output file
• module: single file; can export variables or functions; loaded by templates or other`modules
• ordinary file: single file; read by templates or modules
• package: single directory; contains modules, templates, and ordinary files
• library: collection of packages

A related error message could read:

- cannot load deployment.yml: Expected file 'deployment.yml' to be a loadable
module, but was not (hint: module filename must end with '.mod.yml' or '.star'; use 
data.read(...) for loading ordinary file contents)
    in <toplevel>
      service.yml:2 | #@ load("deployment.yml", "deploy")

[cari-lynn]

We have also been defining a module as either a Starlark module or a YAML module. We have a section on Terminology that these definitions may belong in.

[cari-lynn]

I believe the scope of this issue is to disambiguate .lib.yml (modules) with libraries (ie _ytt_lib) in both the documentation and/or in error messages or user facing messages.

[pivotaljohn]

This kind of thing will get caught up in any proposed revamp of the documentation. No need to have an individual open item tracking it.