Move linear_solvers package from qiskit-terra

[anedumla]

Moving the linear_solvers package from qiskit-terra since it will be deprecated because it is not likely to be used with real hardware in the near future.

[coveralls]

Pull Request Test Coverage Report for Build 3051316696

• 563 of 698 (80.66%) changed or added relevant lines in 12 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.8%) to 87.085%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_research/linear_solvers/observables/matrix_functional.py	44	46	95.65%
qiskit_research/linear_solvers/linear_solver.py	39	42	92.86%
qiskit_research/linear_solvers/observables/absolute_average.py	21	25	84.0%
qiskit_research/linear_solvers/observables/linear_system_observable.py	15	19	78.95%
qiskit_research/linear_solvers/matrices/linear_system_matrix.py	32	45	71.11%
qiskit_research/linear_solvers/numpy_linear_solver.py	9	26	34.62%
qiskit_research/linear_solvers/matrices/numpy_matrix.py	61	83	73.49%
qiskit_research/linear_solvers/matrices/tridiagonal_toeplitz.py	174	198	87.88%
qiskit_research/linear_solvers/hhl.py	151	197	76.65%

Files with Coverage Reduction	New Missed Lines	%
qiskit_research/utils/pulse_scaling.py	3	81.7%

Totals
Change from base Build 3024419879:	-1.8%
Covered Lines:	2987
Relevant Lines:	3430

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💛 - Coveralls