Compiling plugins was originally intended to be an idempotent operation.
This however starts to change as we introduce build customisations,
which have the unfortunate side-effect of changing the state of the
plugin directory, leading to conflicts between concurrent compilation
jobs.
Therefore to mitigate this problem, this commit changes how compilation
jobs are processed, by introducing a global compilation queue, and
processing plugins' compilation one-by-one from this queue.
This however makes such requests asynchronous, so test suites that
require plugins to be compiled will now have to wait on their completion
before they can start their tests.
To this effect, we introduce one more convenience function that
processes those errors, and automatically fails the test should one
compilation job fail for any reason.
The lib name for the common components for writing packer_test suites
was not clear, and did not follow the convention established in Packer
core and plugins.
Therefore this commit does two things: first the lib is renamed into
common as to follow this convention, and clearly document which
components are common to all tests.
Also checkers are placed in a subpackage of common, common/check, so
that it is clearer what is meant to be used as checks for a command's
execution status after it's been run, as part of Assert.
