Build: use experimental 'smartmatch'

It looks like I found a few core bugs (or at least unexpected behaviours):
- "use experimental 'smartmatch'" is not enabled by "use feature
  'switch'"
- "use experimental" is not lexical scoped but package scoped
- "use v5.010" does not enable 'feature switch' and 'experimental
  smartmatch'
MANIFEST.SKIP: ignore more source files

Ignore more files that we mirror from the ARCEP site.