Optimize addonCacheKey computation

[SergeAstapov]

Since adoption of ember-auto-import v2 and ecosystem migration to @embroider/macros@1.0.0 we seen significant performance degradation in our not that small app - it went from 4 mins to 20 mins (but with way to inefficient dependencies graph and too many cross-dependencies, which is separate problem).

Debugging and profiling leads to gatherAddonCacheKey function being biggest contributor.

First issue
Every time @embroider/macros addon is included (and it's included way more than few times due to ember-auto-import and many other addons depend on it) the whole tree travels happens over and over.

Second issue
there is return [...memo].join('|') line which runs for each end every recursion level but the result is used only from top-level return when invoked with project as an argument.

Changes in this PR propose to traverse addons three only once and build cacheKey string only once per project.

I wasn't sure if project always is the same reference or there are use cases when we may have more than one project during build time hence used WeakMap.

If we know for sure project always is the same reference - than we can replace WeakMap with simple string variable that would be returned once we invoke gatherAddonCacheKey second and subsequent times.

[ef4]

Thanks, very nice.