The dir component ("somedir" in #include <somedir/fo...>) is considered fixed.
We append "foo" to each directory on the include path, and then list its files.
Completions are of the forms:
<somedir/foo.h>
^-text--^^-TT-^ (TypedText)
and
<somedir/foodir/
^-text--^^-TT--^
The filter is set to the filename part ("fo"), so fuzzy matching can be
applied to the filename only.
No fancy scoring/priorities are set, and no information is added to
CodeCompleteResult to make smart scoring possible. Could be in future.
Note directory iteration is currently stat-happy on unix but D51921 will fix.
It's also valid to use \ for path separators on Windows, maybe also handle those?
Clang also supports this when running e.g. clang-cl /c foo.cpp.