build scripts: factor out configure.sh; start using .build directory

configure.sh is responsible for probing the system, downloading
dependencies, and building dependencies. It's OK to be slow, because
it should run rarely. build.sh builds Fubsy, so it should be fast.

Both of these scripts should write build products to .build/1, the
"stage 1" build directory. Stage 2 will be for the self-hosted build,
i.e. using the "fubsy" binary from stage 1 to build itself. Not quite
there yet!
add barebones plugins package, just enough to embed Python

All it can do right now is run a bit of Python code: there's no access
to data back-and-forth, i.e. Fubsy cannot access Python code nor can
Python code access Fubsy internals. But hey, it's a start!
dsl: make syntax for inline plugins nicer; trim common leading space

Require newline right after the "{{{" and before the "}}}", and strip
both of those newlines. That means the following are legal:

  plugin x {{{
  }}}
  plugin x {{{
  content
  }}}
  plugin x {{{
    indented content
  }}}

but these are not:

  plugin x {{{}}}
  plugin x {{{content}}}
  plugin x {{{
    content}}}
plugins: update import path for go-python
Needed because of upstream commit ea8c7d1e04a8a7d14dc5720c2eee5424ca7aa7e5