I have ctfe generated code which requires generating import
statements so symbols can be looked up(avoiding the need to have
to manually import modules).
1. Allow identical import statements not throw an error.
import std.conv : to;
import std.conv : to;
throws error
Error 1 Error: alias main.to conflicts with alias main.to at
main.d(5)
such errors make it difficult to write generated code with weak
state information(since ctfe's can store global state data.
2. Allow if an isImported statement which checks if a symbol is
imported. This can reduce unnecessary import statements.
I get around 1 by using a table to store the import statements,
but, again, this only works inside the ctfe call since the table
can't be global. Multiple code generation in the scope may
produce the issue.
