Makefile: refactor lookup of pkg build rules and deps

Introduces a LOOKUP_PKG_RULE function that matches generic rules (for BUILD or DEPS) from targets specified along the lines of:

triplet.lib-type.debug.treading.exceptions.my.experimental.version

Current build rules will be matched according to the current logic (pkg-target --> pkg) and future variants can use "targets" as both functionality determinants and a directory separator for multiple side-by-side variants.

Also replaces the `no-op` and `exclude` notices with a single `no-build`, though I'm not sure that is even necessary and would like to only notify when a build is actually occurring (I added those notices, but don't find myself looking at them). The only case I can think of where a `no-build` message would be useful is if that package was explicitly specified on the command line.