Fix file search (gf, [I, etc: path & suffixesadd settings)

[softmoth]

Closes #12

This is a stripped down and improved version of the patch in #12, so credit belongs to @eiro and @zostay for the main idea.

After this patch, 'gf' for example will search:

• ./lib
• .
• $RAKULIB
• ~/.zef/store///lib (sorted in reverse directory mtime)

A search for Foo::Bar will look in those lib dirs for one of Foo/Bar.{rakumod,rakudoc,pm6,pm}.

[zostay]

Thanks.