* perf(rtp): reduce rtp scans
Problem:
Scanning the filesystem is expensive and particularly affects
startuptime.
Solution:
Reduce the amount of redundant directory scans by relying less on glob
patterns and handle vim and lua sourcing lower down.
Problem: Bashslashes added as regexp in runtime completion may be
treated as path separator with some 'isfname' value.
Solution: Make curly braces work for runtime completion and use it.
This ensures that colorschemes in 'rtp' are tried before ones in 'pp',
because some colorschemes in 'pp' may not work if not added to 'rtp'.
This also match the current documentation better.
