[Go] Fix bug where bytes wasn't being imported when using --gen-onefile flag

[le-michael]

Fix for discussion in #7460.

Currently, the go generator resets the state of needed imports each time we process an enum or struct. This works fine when we want each enum and struct to be in their own files.

However, this will lead to an error if an fbs file is defined like the following.

namespace example;

table Foo {
  name: string (key);
}

table Bar {
  id: int;
}

When processing the Foo table the needs_bytes_import will be set to true because of the key field. Then when processing the Bar table the needs_bytes_import will be reset to false resulting in the final output file to not import the bytes package.

When we want all code to be in the same file we shouldn't reset the needed imports as we iterate through the structs/enums.

Additional Changes
For the lookup function I moved the declaration of bKey outside of the for loop to reduce unnecessary casting.