NAG: Fix using Fortran modules from their output directory

The NAG Fortran compiler's `-mdir` flag sets the module output
directory but does not add the directory to the search path for using
modules.  This is inconsistent with other compilers like the GNU Fortran
compiler's `-J` flag that does both.  In order to make these consistent,
add the module output directory with a `-I` flag on the NAG Fortran
compiler so that it will be searched when using modules too.

We already do this for the XL Fortran compiler since commit 210b0b99
(XL: Fix using Fortran modules from their output directory, 2020-02-28,
v3.18.0-rc1~640^2~1).
31 jobs for !5842 with nag-fortran-include-moddir in 4 minutes and 47 seconds (queued for 1 second)
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linux-3.17 linux docker build cmake
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x11 linux-3.17 linux docker cmake
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nonconcurrent msvc-19.28 vs2019 shell windows cmake
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linux docker cuda-rt cmake
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linux-3.17 linux docker build cmake
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