Use absolute imports rather than relative

Apparently this is in general preferable, and certainly the bulk of such
pysam imports (71 vs 3) are already "pysam.libcfoo".

pysam/libcbcf.pyi

@@ -20,7 +20,7 @@ if sys.version_info < (3, 8):
 else:
     from typing import Literal
 
-from .libchtslib import HTSFile, _HasFileNo
+from pysam.libchtslib import HTSFile, _HasFileNo
 
 _D = TypeVar("_D")
 _K = TypeVar("_K", str, Union[int, str])

pysam/libcutils.pyx

@@ -20,10 +20,10 @@ from libc.stdio cimport stdout as c_stdout
 from posix.fcntl cimport open as c_open, O_WRONLY
 from posix.unistd cimport SEEK_SET, SEEK_CUR, SEEK_END
 
-from .libcsamtools cimport samtools_dispatch, samtools_set_stdout, samtools_set_stderr, \
+from pysam.libcsamtools cimport samtools_dispatch, samtools_set_stdout, samtools_set_stderr, \
     samtools_close_stdout, samtools_close_stderr, samtools_set_stdout_fn
 
-from .libcbcftools cimport bcftools_dispatch, bcftools_set_stdout, bcftools_set_stderr, \
+from pysam.libcbcftools cimport bcftools_dispatch, bcftools_set_stdout, bcftools_set_stderr, \
     bcftools_close_stdout, bcftools_close_stderr, bcftools_set_stdout_fn
 
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