liblink: don't search for an import file with an absolute path
liblink: don't search for an import file with an absolute path

This restores the old behaviour, and makes it possible to
continue to use 6g and 6l directly, rather than the go tool,
with dot imports.

R=rsc
CC=golang-dev
https://codereview.appspot.com/43710043

tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme