revised reflection handling

[lu4p]

(see commit message)

[mvdan]

SGTM - looks a lot better without all the pkgcache changes. I'm still mildly worried about the internal/abi performance but we can measure and improve that once it hits master. Please revert the change to the hashed name lengths though, as described.

[mvdan]

Also, how about this for a more detailed commit message:

obfuscate all names used in reflection

Go code can retrieve and use field and method names via the `reflect` package.
For that reason, historically we did not obfuscate names of fields and methods
underneath types that we detected as used for reflection, via e.g. `reflect.TypeOf`.

However, that caused a number of issues. Since we obfuscate and build one package
at a time, we could only detect when types were used for reflection in their own package
or in upstream packages. Use of reflection in downstream packages would be detected
too late, causing one package to obfuscate the names and the other not to, and a failure.

A different approach is implemented here. All names are obfuscated now, but we collect
those types used for reflection, and at the end of a build in `package main`,
we inject a function into the runtime's `internal/abi` package to reverse the obfuscation
for those names which can be used for reflection.

This does mean that the obfuscation for the names is very weak, as the binary
contains a one-to-one mapping to their original names, but they cannot be obfuscated
without breaking too many Go packages out in the wild. There is also some amount
of overhead in `internal/abi` due to this, but we aim to make the overhead insignificant.