IN 2002, YOU COULD HAVE SEEN EMILY JOHNSON’s Plain Old Andrea, with a Gun at the Southern Theater as part of the Momentum series. In 2004, you could have seen her Heat and Life at the Soap Factory, presented by the Walker. But since then, you’d have to be either lucky or savvy to catch Johnson — she’s performed primarily at various small venues (the BLB, the Rogue Buddha Gallery) or in site-specific…