cmd: Fix eval subcommand to remove blank lines from partial output

The topdown package re-uses location information from AST nodes that
it saves and the format package attempts to squash blank lines between
adjacent expressions. As a result, the pretty/source formatted output
from the eval subcommand would often contain spurious blank lines that
make the output feel inconsistent.

This change modifies the cmd package to overwrite the location info on
all expressions so that the format package doesn't have to squash any
blank lines. The cmd package walks the expressions and sets the row
number in-order.

Before:

+----------+------------------+
| Query 1  | "R1" = input.foo |
|          |                  |
|          | "L1" = input.bar |
+----------+------------------+

After:

+----------+------------------+
| Query 1  | "R1" = input.foo |
|          | "L1" = input.bar |
+----------+------------------+

Signed-off-by: Torin Sandall <torinsandall@gmail.com>