This changes the representation of maps in the interpolator from the
dotted flatmap form of a string variable named "var.variablename.key"
per map element to use native HIL maps instead.
This involves porting some of the interpolation functions in order to
keep the tests green, and adding support for map outputs.
There is one backwards incompatibility: as a result of an implementation
detail of maps, one could access an indexed map variable using the
syntax "${var.variablename.key}".
This is no longer possible - instead HIL native syntax -
"${var.variablename["key"]}" must be used. This was previously
documented, (though not heavily used) so it must be noted as a backward
compatibility issue for Terraform 0.7.
A consequnce of the work done in #6185 was that variables which were in
a module but not set explicitly (i.e. the default value was relied upon)
were marked as type errors. This was reported in #6230.
This commit adds a test case for this and a patch which fixes the issue.
These tests demonstrates a problem where the types to a module input are
not checked. For example, if a module - inner - defines a variable
"should_be_a_map" as a map, or with a default variable of map, we do not
fail if the user sets the variable value in the outer module to a string
value. This is also a problem in nested modules.
The implementation changes add a type checking step into the graph
evaluation process to ensure invalid types are not passed.
