lib/n/builtins: shorten file names
lib/n/builtins: Error: call member 'Why', error constants all UC

We will likely want to make 'Why' a ?@`Show, but for now it's easier
with a plain String.
bootstrap: support for relative imports

Normal imports are looked up in a well-defined list of prefixes
(currently $PWD and $PWD/lib).

Until now, we also looked in the directory where the importing module
was found. Now, a relative import must be used (and it only looks in
that directory, not in the global list of prefixes).

	import .relative
	import .a.b
trun, taccept, tdiff: lookup for tests the same way; do not filter on '_'

Filtering on '_' is unnecessary now that multi-modules files are found
in directories.
trun: use prefix 'skipped_' to have a file in t??/ not be considered a test
bootstrap:parser: parse_modpath: bogus comment: mod lookup doesn't allow 'a.linux.n'

This was a bit of an ill-formed idea expressed in a comment a while
back. It doesn't make sense in the current form of the module system.
bootstrap:import: change import naming: 'import a.b.c' imports under 'c.Name'

This is a much nicer rule than previously, when we created local,
complete names 'a.b.c.Name'. Modules names are great to avoid repetition
in naming, but we typically don't need the whole module path. If
ambiguities arise, we can use a 'import a.b.c as d' form (TODO: add
support for it).
lib/n/syscall: Pread, Pwrite; fix bug in lseek wrapper