Integrate "available" to check for "valid" package names

[jorainer]

Would be nice to integrate the available package as an additional check to BiocCheck to check against problematic package names.

[LiNk-NY]

I created an avail branch for testing.
https://github.com/Bioconductor/BiocCheck/tree/avail/
It's still a work in progress but the output looks a bit unwieldy.
I may decide to drop the functionality if the output does not look
amenable to BiocCheck's style of output.