The os.path.realpath() function is expensive and we call it many times,
to the point that os.path.realpath() calls make up around 40% of the
total time spent in Element.stage_artifact().

The cleanest way to fix this is with a `functools.lru_cache()` wrapper
that caches recently used values. None of the code in question can be
removed (as the tests added in the previous commit will demonstrate).

I tested this by running `bst shell base/base-system.bst true` in
the GNOME modulesets project.

  o Without this patch there are 240,019 calls os.path.realpath()

  o With this patch there are 10,379 calls to os.path.realpath()

  o If we increase the cache size to 128 items, there are 10,359 calls
    to os.path.realpath().

  o If we reduce the cache size to 32 items, there are 10,426 calls.

  o In all cases the number of *unique* calls is 10,327.

This fixes issue #174.

Before:

    [--:--:--] WARNING Failed to fetch remote refs from ssh://ostree@ostree.baserock.org:22000/cache: BuildStream did not connect successfully to the shared cache ssh://ostree@ostree.baserock.org:22000/cache: SSH error: ssh: connect to host ostree.baserock.org port 22000: Connection refused

After:

    [--:--:--] WARNING Failed to fetch remote refs from ssh://ostree@ostree.baserock.org:22000/cache. ssh: connect to host ostree.baserock.org port 22000: Connection refused

This does not introduce a performance gain alone, but importing pkg_resources
is costly at the moment and can be avoided until it is actually needed.

A LocalPath was being passed into os.path.isabs, which failed when it
tried to call the path's startswith method.

Converting it to a string before using _yaml.dump seemed to solve the
problem.

Do not recalculate the shared part of cache keys. In particular, do not
call get_unique_key() multiple times for a single element or source.

Fixes #167

This adds the _update_state() method to the Element class to keep track
of element state and avoid calculating the same cache key multiple
times. This also consolidates the different get_cache_key
methods into a single method that always returns the cache key
calculated by _update_state(), if available.

Fixes #149, #173

Originally this was created with also `bootstrap-commands` and
`test-commands` but these were never documented or used.

For `bootstrap-commands`, these originated in baserock but are
basically a part of the `configure-commands` stage, prepending
commands to `configure-commands` is more suitable here since we
now have the prepend/append list directives.

For `test-commands`, these were never used and it's unclear at
this time if it's the correct place for it. It would be interesting
to implement `test-commands` as a separate operation which can run
in parallel with reverse dependency builds (no need to block the
build of a reverse dependency on failing tests, we can still fail
the build as a whole based on a failing test without blocking
builds).