Infer proper package names

[bmatsuo]

Gonew should be smarter about package (target) names for "pkg" commands so that supplying the -target flag is unnecessary in most situations. Specifically, it should make the following assumptions:

• Projects named "go-foo" should contain a package named "foo"
• Projects named "bar.go" should contain a package named "bar"

Nothing special should be done for projects named "gobaz". It seems to me the most common practice for projects with these names is for the package to be retain the "go" prefix.