Also added methods to Load and Save the cache from disk without the need
to pass cacheFilePath back and forth.

...ner_find_includes.go
There is no point in keeping those structures inside module types
because:
- they are used only in 'container_find_includes.go'
- the entry in Context is used only in for library discovery
This commit prepares for the extraction of the library discovery logic
from legacy package.

- Added 'Contains(...)' method (instead of sliceContainsSourceFile(...))
- Removed useless comparators

All the fields needed to perform library discovery have been moved into
the CppIncludeFinder so there is no more need to pass them around as
parameters making it much more lightweight.

Previously the Origin interface{} field was used to store the "origin"
of the source file (being sketch or library). Actually Origin is really
needed only to reconstruct BuildPath and RootPath when the origin is a
Library, in all the other cases we can reconstruct BuildPath and RootPath
looking inside Context.
With the above in mind, this commit removes the non-idiomatic
interface{} by replacing it with a Library, so now the meaning of the
SourceFile.Library field is "the library where the source file is
contained or nil if not part of a library".

I don't know the reason why it was not placed here already, the git log
doesn't give much hints. Anyway this change prepares for the next commit
where the Library field is being removed from SourceFile struct.

...ters

...rence

... method

This is a series of commit to make library detection context-agnostic.
In this commit the IncludeFolders usage has been removed from the
internal logic, a new field CppIncludesFinder.IncludeDirsFound has been
made for this purpose. The new field is setup before by the caller
before doing the actual discovery.