Make IdrisDoc namespace tracing strict

[PhilipBorgesen]

IdrisDoc is not strict enough when it comes to tracing which namespaces should be included in the generated documentation.
An example is the idrisdoc004 test (pull request #1103) where [builtins] and in turn several other Prelude package namespaces are included, although no visible reference is made to them.

I believe it is caused by the 'extractPTermNames' function found in Idris.IdrisDoc, which is too greedy in its tracing of referred namespaces. It returns the namespaces of all names recursively encountered in a PTerm. Because of a lack of understanding of what each PTerm is used for, I am not able to filter out the unnecessary cases myself.