lsif-semanticdb should take --semanticdbDir as an argument(s)

[wiwa]

$ lsif-semanticdb META-INF/semanticdb/foo
error: unexpected META-INF/semanticdb/foo

$ lsif-semanticdb --help
usage: lsif-semanticdb --semanticdbDir=SEMANTICDBDIR [<flags>]

I believe it would be a better experience if semanticdbDir were an argument instead of a required parameter, i.e. passing it the semanticdbDir directly so we could use it with the syntax lsif-semanticdb META-INF/semanticdb/foo.