Optimize performance for binder, imports and packages (Rebased from sbalabanov/master)

[StevenACoffman]

This is not my work. I just rebased #1702 against master.

This PR consists of multiple commits and is pure refactoring. It should maintain full backwards compatibility with the origin.

• imports.goModuleRoot is called for every type resolution. It tries to find go.mod file in the directory tree making a gazillion of requests to open file. While someone more familiar with the application design can come up with a better architecture to avoid this altogether, I added a caching layer on top of this function to only traverse file system once.
• binder.FindObject traverses all definitions in a package and for all needed types it is effectively O(n^2). Make it to O(n) by caching all package top-level definitions in a hashtable.
• packages.Load calls 'packages.LoadAll' which allocates a slice from the heap even when a single package is required. Added a fast path by checking a cache map before calling to packages.LoadAll to avoid that. A possibly better solution is to refactor common parts of packages.Load and packages.LoadAll into a separate function.

For the targeted use case we achieved ~20% speed optimizations after those improvements.

Because functionality did not change, existing tests should cover it all so no new tests added.

[coveralls]

Coverage increased (+0.2%) to 67.578% when pulling 24ed400 on sbalabanov/master into 237a7e6 on master.