dts: dtlib: Refactor to get rid of _is_parsing flag

The public DT.get_node() function was used during parsing to look up
paths in references like &{/foo/bar}, along with an ugly
'DT._is_parsing' flag to adapt its behavior (to not mention aliases in
error messages).

Split out common node lookup code needed during parsing and by
get_node() instead, and stop using get_node() during parsing. This
allows '_is_parsing' to be removed and untangles things a bit.

Piggyback some other small reference-related cleanups, and fix an issue
with the filename/linenr being given twice in some error messages.

This commit also removes the index of path components from error
messages, but just the string is probably good enough.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>