Rename the cache/dlprefix/outdir/output arguments

[kwinkunks]

The same concept, a temporary directory for pip and other build tools to download into, has several names:

• cache in cli.py (and therefore in the kmd CLI)
• dlprefix in build.py
• outdir in komodo.fetch.fetch()
• output in the CLI implemented by komodo.fetch.py.

So there are at least 4 names for the same thing. This is confusing for the developer, but could lead to confusion for the user too, for example if they pass cache to the CLI, then it complains of a problem with outdir emitted by fetch.py.

It's going to need a DeprecationWarning because who knows who/what is using these various functions.

[kwinkunks]

See also #125