Bad error message for missing module in --make

GHC --make uses getImports to find the imports of a 
module.  It has two bad flaws

a) it's a total hack, parsing the file character by 
character, which is really slow.   We though it stopped 
after the import statements but it doesn't; it chunters 
through the entire file

b) It does not record line numbers, so if the imported 
module does not exist, there is no decent error message

We have a plan; this bug report makes sure we don't 
forget.

Trac metadata

Trac field	Value
Version	6.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	ResolvedFixed
Component	Compiler
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