staticcheck: improve and enable CheckSillyRegexp

[dominikh]

CheckSillyRegexp flags regular expressions that do not contain any meta characters. Such regexps can often be replaced with simpler string functions. The reason the check isn't currently enabled is that the user may be required to use a regexp because they're using a third party API that only accepts regexps.

We should improve the check (some basic data flow analysis to see if the regexp flows into a regexp matching function in the same package), and possibly make it a quickfix check instead of a staticcheck one.