Before golang.org/cl/13921, "go install -buildmode=shared prefix/..." created a
file called "libprefix.so", which was obviously a problem when prefix was
something like "." or "../".  However, now it expands the ... into all the
matched packages, joins them with -, which can clearly be a very long name
indeed. Because I plan to build shared libraries for Ubuntu by running commands
exactly like "go install -buildmode=shared prefix/...", this special cases this
to produce the old behaviour (but de-relativises prefix first).

Fixes #13714

Change-Id: I4fd8d4934279f9a18cc70a13e4ef3e23f6abcb6e
Reviewed-on: https://go-review.googlesource.com/18114Reviewed-by: Ian Lance Taylor <iant@golang.org>