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Initializes a PathList object, canonicalizing the input and
pre-processing it for quicker substitution later.
The stored representation of the PathList is a list of tuples
containing (type, value), where the "type" is one of the TYPE_*
variables defined above. We distinguish between:
strings that contain no '$' and therefore need no
delayed-evaluation string substitution (we expect that there
will be many of these and that we therefore get a pretty
big win from avoiding string substitution)
strings that contain '$' and therefore need substitution
(the hard case is things like '${TARGET.dir}/include',
which require re-evaluation for every target + source)
other objects (which may be something like an EntryProxy
that needs a method called to return a Node)
Pre-identifying the type of each element in the PathList up-front
and storing the type in the list of tuples is intended to reduce
the amount of calculation when we actually do the substitution
over and over for each target.
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